Abstract: An improved power management circuit for regulating current on a loop converts a high voltage, low current loop circuit to a low voltage source for devices associated with the loop. Additionally, the improved power manager provides a port for frequency shift key (FSK) transmit and receive signals.

Abstract: A ring-trip circuit monitors the voltage developed across a source resistor coupled to one of the leads of the subscriber line circuit, regardless of the ringing frequency of the ringing voltage applied to the line circuit, and irrespective of the polarity of the DC loop current developed in the line. In response to detecting a prescribed threshold level of the source resistor voltage--indicating that the customer has answered the call, the ring-trip circuit provides a digital output that is read by the control equipment of the central office to terminate the ringing voltage.

Abstract: When two or more telephone sets are in use on the same subscriber's loop, their activities may not be properly detected or monitored by the central office switch. The present invention provides a method of and an apparatus for coordinating activities so that call waiting caller ID or off-hook caller ID or other service features can work even if there are off-hook extensions on the same loop. The invention ensures that only one set among a plurality of off-hook sets responds to the switch.

Abstract: A telephone device for preventing a long-term on-line state resulting from the handset being unintentionally left off-hook.

Abstract: A telephone ring signal detector for detecting a telephone ring signal voltage on a telecommunications line connectable to at least one line telephone device. Detector includes a ring signal processor comprising a voltage translating device for providing a translated electrical parameter and an electrical parameter threshold setting device for providing a predetermined threshold reference electrical parameter value. The setting device provides a ring signal detect output signal when an electrical parameter value of the translated parameter exceeds the predetermined threshold reference electrical parameter value in a first quarter cycle time period of a first telephone ring signal cycle time period.

Abstract: An apparatus and a method of setting and confirmation of a value indicative of an on-hook period of a terminal is provided. The value is used as a threshold when distinguishing a hooking pulse for putting communication on hold which is issued by the terminal, from disconnection of a call on the terminal.

Abstract: A method and apparatus for screening an incoming telephone call from a caller on an external telephone line connected to an answering device for recording the call includes inserting into the telephone line a circuit comprising a voltage sensitive device and a bridging device for allowing a handset of a telephone set connected to the telephone line and remote from the answering device to be picked up to listen to the call. The handset of the telephone set is picked up to enable a speaker component of the telephone set without interrupting operation of the answering device. The incoming call is listened to via the enabled speaker component while the answering device operates. A microphone component of the telephone set is optionally enabled to speak with the caller. The operation of the answering device may be interrupted by pressing a switch.

Abstract: A power failure shunting office line restoring circuit includes: a first relay having an office line tip port connected to a common port for connecting the tip port of the office line to a separate emergency telephone or a key telephone office line accommodating circuit in accordance with a first relay control signal; a second relay having an office line ring port connected to a common port for connecting the ring port of the office line to the emergency telephone or the key telephone office line accommodating circuit in accordance with a second relay control signal; a controller for generating the first and second relay control signals according to a power supply state or emergency telephone use state detection signal; first and second relay drivers for driving the first and second relays in accordance with the first and second relay control signals generated by the controller, and a detector connected to the first relay for detecting the use of the emergency telephone and for outputting the use state detect

Abstract: A loop detector circuit for detecting the loop current of a telephone circuit in a ground start mode and a normal mode which obviates the need for a separate TIP ground detector circuit. A pair of light emitting diodes (LED) 52, 54 are associated with a photoelectric device, such as a phototransistor 56, 58, connected across TIP and RING with a series resistor. Each resistor 60, 62, is selected to provide a predetermined voltage drop during a ground start mode and a normal mode, thus enabling the loop detector to detect loop current during both modes. The loop detector circuit is thus able to detect the off-hook status of a subscriber handset during a ground start mode without the use of a separate TIP ground circuit.

Abstract: A reduced complexity method of ring signal detection, in which a digital signal processor monitors an incoming data stream for zero crossings and employs a limiter-discriminator to differentiate ring frequency signals from line noise. Controlled noise is injected into the incoming data stream in order to prevent false declarations of ring signals in the presence of low frequency natural noise or low amplitude signals. The method conserves valuable processing power and may therefore prove useful in DSP-based telephone devices that must simultaneously perform multiple processing operations such as ring detection and speech processing.

Abstract: A standby feed circuit for a subscriber line comprising a constant current power feeding circuit for a subscriber line for supplying a voltage to a subscriber line at a voltage less than a standard central office power feeding circuit, a voltage source for providing the standard voltage, apparatus for applying the standard voltage when a station apparatus connected to the subscriber line is in an on-hook condition, and apparatus for applying said voltage less than the standard voltage in place of the standard voltage when the station apparatus is in an off-hook condition.

Abstract: A communication loop using a subscriber line interface circuit (SLIC) is modified such that it can operate in unbalanced ringing environments as well as balanced ringing environments. This is accomplished by grounding the tip lead of a telephone instrument and creating an open circuit at the tip lead of the SLIC. The ring lead of the telephone instrument and the ring lead of the SLIC are electrically connected, and an AC voltage applied to the ring lead by the SLIC is increased. A power supply is connected to the SLIC and biased to provide the increased AC voltage on the SLIC ring lead while the average voltage on the SLIC tip lead is maintained substantially at ground. The result of this circuitry is a trapezoidal signal meeting the amplitude, loading and crest factor requirements of Bellcore Technical Requirement Publications TA-NWT-000909 and TR-NWT-000057.

Abstract: A switch hook interface circuit couples a subscriber telephone to a subscriber telephone line. This interface circuit comprises a source of an off-hook signal, having one state indicating an off-hook condition, and a second state indicating an on-hook condition. A circuit is coupled between the off-hook signal source and the subscriber telephone line which gradually increases the current drawn from the subscriber telephone line when the off-hook signal indicates an off-hook condition, and gradually decreases the current drawn from the subscriber telephone line when the off-hook signal indicates an on-hook condition.

Abstract: A transformerless battery feed circuit for a digital telecommunications line (100) presents a high impedance to digital signals and a low impedance to direct current, and provides a constant-voltage feed with current limiting. The battery feed circuit comprises a pair of transformerless controlled current sources, one for each lead (T,R) of the line, each taking the form of a high-gain transistor (Q1, Q2) having a collector coupled to a source of power (e.g., a battery V1), a base resistively coupled to the power source and capacitively coupled to its corresponding lead, and an emitter coupled to its corresponding lead. The battery feed circuit further comprises a pair of control circuits, one for each current source, each taking the form of a transistor (Q3, Q4) having a collector coupled to the base of the corresponding high-gain transistor, a base resistively coupled to the emitter of the corresponding high-gain transistor and to the power source, and an emitter coupled to the corresponding lead.

Abstract: A line circuit provides BORSCHT functions and supplies the customer loop through a feed resistor network and current limiter either from a high voltage battery to provide for maximum signaling range or from a lower voltage battery to conserve power. The current limiter operates under the control of a microprocessor/DSP which selectively applies or denies power to isolate or include components for testing and controls the switching in and out of transistors to control the scaling factor between the channel width-to-length ratios of the current mirror's input and output FET transistors according to the type of test or operating condition dictated by the microprocessor while the input reference current to the current mirror is maintained constant.

Abstract: A method and apparatus provide communication in a telephone system connected to an external phone line, wherein the telephone system has a telephone line, a telephone, a phone interface unit coupled to the telephone and the telephone line, a computer system, and a computer interface unit coupled to the computer system and the telephone line. A bias voltage on the telephone line is detected to determine if the telephone is on-hook or off-hook, a ringing voltage on the telephone line is detected to determine if an external call is incoming, and a first carrier transmitted by the phone interface unit is detected. A second carrier is transmitted using the computer interface unit under control of the computer system. Detection of the bias voltage, ringing voltage, and/or first carrier is communicated to the computer system. An off-hook condition is indicated if the line bias voltage is less than an off-hook bias threshold voltage.

Abstract: A subscriber line circuit which truly implements all of the BORSCHT functions and which does not rely on an external ringing generator to provide the ringing voltage when needed is disclosed. This implementation is achieved by recognizing that supervisory and ringing signals occur in some sort of sequence and, therefore, do not have to be generated simultaneously. Based on this recognition, we provide a circuit which truly implements all of the BORSCHT functions and includes a power section that generates each of the supervisory signals, e.g., -48 volts and -24 volts, and alerting signal, e.g., ringing signal, in turn and as required.

Abstract: A telephone apparatus includes a phone jack arrangement (299) providing an interface to land lines from the central office and other telephone apparatuses. Phone jack arrangement (299) comprises a phone jack (306) having inner conductors (420, 422) coupled to outer conductors (406, 408) of a phone jack (304), and outer conductors (424, 426) coupled to inner conductors (402, 404) of phone jack (304). Condition detectors (232, 234) detect whether phone lines of phone line circuitry (270) are connected to land lines, and microprocessor (212) may operate the telephone apparatus or configure base station user interface (126) accordingly. A two-line operating mode may select one of the phone lines for operating use upon detection of a predetermined condition from condition detectors (232, 234) or ring detectors (246, 248). Thus, a multi-line telephone apparatus operates flexibly according to different phone line connections.

Abstract: An event detector for a headset connected to a telephone, comprising a switch for short circuiting a microphone of the headset and a circuit for detecting a hookswitch flash in the event the duration of the short circuiting of the microphone is greater than a predetermined minimum and less than a predetermined maximum duration.

Abstract: An apparatus and method for determining the presence and polarity of DC bias voltage at a telephone microphone. A circuit can be added in parallel between any hand or headset and a telephone base unit. The circuit converts the DC bias voltage, if any, across the microphone into a pulsed signal. Each pulse charges a capacitor. The output of the capacitor is passed through a resistor transformer network, and sampled. In between each pulse, the capacitor is discharged and the resulting signal is passed through the transistor/resistor network and sampled. By determining the averages of the charging and discharging signals, and sampling them to convert it into digital information, a digital signal processor can derive whether any bias voltage is present across the microphone, and, if so, its polarity. The circuit can be electrically isolated from the microphone and telephone.

Abstract: A dial pulse detector in a paging system for rendering a paging service for a subscriber who uses a mechanical telephone includes a delay correlativity calculator for receiving dial pulses generated by predetermined pre-test dialing digits dialed by the subscriber, calculating the correlativity between the delayed dial pulse and undelayed dial pulse to sample a reference signal of the dial pulse. An energy correlativity calculator calculates the energy correlativity between the sampled reference signal and the successively received dial pulse to examine the validity of the reference signal. A dial pulse verifier operates according to recognition of the validity of the reference signal by the energy correlativity calculator and checks input number of times and input time of the dial pulse so as to verify whether the dial pulse corresponds to the pre-test dialing digits.

Abstract: A communications interface having an isolation circuit for signals from a plain old telephone set (POTS) device to communications electronics. The communications interface having an isolated power supply. The isolated power supply including a flyback transformer for a current loop supply and an isolated power source The telephone line isolation circuit having an optically isolated off-hook detection circuit and an isolation transformer for isolating communication electronics from the telephone current loop. The telephone line isolation circuit providing a ringing circuit for ringing a current loop device, such as a standard plain old telephone system (POTS) device.

Abstract: A line interface circuit (LIC) used in telecommunications networks operates in either a high frequency mode or a low frequency mode, depending upon the status of its associated customer premises equipment. A LIC main controller detects a change in state of customer premises equipment via a battery feed circuit. If the customer premises equipment is off-hook, a switch associated with a resistor-capacitor (RC) circuit is activated to lower the resistance value of the RC circuit, and thereby, increase the operating frequency of the LIC. During on-hook intervals, the main controller deactivates the switch, and the LIC operates at a lower, default frequency.

Abstract: A circuit and method for maintaining switch-hook detection (SHD) in the presence of a fault in a telephone system may include a comparison of one of a tip current and a ring current to a reference current. A switch-hook detection indicates that a request for service has been made. The reference current may be a conventional switch-hook detection threshold current, I.sub.SHD, times a factor (G.sub.0 +1).sup.-1, where G.sub.0 is the predetermined operating gain of the amplifier in the telephone subscriber line interface circuit. The line current may be reduced in response to detection of a fault in the telephone system without affecting the determination whether a request for service has being made.

Abstract: A communication terminal device having a line voltage detection circuit 6 connected in parallel with telephone lines 2 is characterized that when either of a built in telephone set 4 or an auxiliary telephone set 5 is off-hooked and then the hook detection circuit OH1/OH2 of the that telephone set 4/5 becomes on-hooked, the true on-off-hook states of that telephone set 4/5 and the branch telephone set 3 are judged by referring to the hook detection circuit OH1/OH2 and the aforementioned line voltage detection circuit 6 after a predetermined time period T has elapsed.The communication terminal device such as a facsimile device and the like having the arrangement described above is able to detect the off-hook/on-hook state of a built in/auxiliary telephone set connected to the telephone lines in parallel with a branch telephone set without mistake even if the branch telephone set is off-hooked.

Abstract: A power cross circuit detects a ring trip condition in order to switch a pair of switch relays to a protection circuit and to appropriate line driver circuitry to enable a talking condition for a telephone. The power cross circuit detects a rise in a DC current across a feed resistor connected to a ring bus and a ring line. The power cross circuit determines a difference in current passing across a first and second sense amplifier connected to respective ends of the feed resistor. The current difference is then converted to a voltage difference, and utilized by the power cross circuit to output a power cross condition when the current difference exceeds a predetermined value.

Abstract: A telephone interface circuit with impedance termination during call pauses, comprising first and second terminals to connect to a telephone line and a line termination branch connected to the first terminal across a decoupling capacitor, and to the second terminal. The line termination branch includes an intersection node, a first switch and a first impedance resistor between the first terminal and the intersection node, and a second switch and a second impedance resistor between the second terminal and the intersection node. The intersection node is connected to ground across a third switch. The first switch, the second switch, and the third switch are controlled by a control unit by first, second and third control lines, respectively. The telephone interface circuit further comprises a ring signal detector that is connected to a microprocessor unit capable of activating the control unit in an interval in which the circuit must be terminated for CLID (Calling Line IDentification) transmission.

Abstract: An apparatus for muting communications subscriber equipment connected to a communications line includes a monitoring circuit for monitoring electrical conditions of the communications line, when the subscriber equipment is connected thereto. A disconnection circuit is provided for disconnecting the subscriber equipment from the communication line and a termination circuit is provided for terminating the communications line with a dummy termination when the subscriber equipment is disconnected, such that, the communications line appears to be terminated by the subscriber equipment. When the subscriber equipment is disconnected from the communications line a power supply circuit provides power to the subscriber equipment with the same electrical conditions as determined by the monitoring circuit prior to disconnecting the subscriber equipment.

Abstract: A communications device is presented which is able to detect a ring signal without the need for dedicated ring signal detection circuitry. The communications device includes a hookswitch coupled between a telephone line and a communications circuit. In an open mode of the hookswitch, an electrical resistance of at least 25 megohms exists between two switch terminals of the hookswitch, and the communications circuit is essentially disconnected from the telephone line at d.c. A capacitor positioned between the switch terminals of the hookswitch couples a.c. signals from the telephone line to the communications circuit when the hookswitch is in the open mode. A DSP of the communications circuit performs ring signal detection when the hookswitch is in the open mode using electrical power provided by a power supply. The input impedance of the communications circuit is substantially equivalent to a series combination of an inductance and a resistance.

Abstract: A system and method for analyzing audio signals sampled from a telephone line during moving sample windows in order to detect indicia of three way calling attempts. The system and method sample audio signals on the telephone line, sort the sampled signals into a profile of levels for the sampled audio signals, and monitor the profile of sampled audio signals for reset and reference conditions. Reset conditions are pulse patterns identified from sampled audio signals that are inconsistent with patterns generated by three way call events. Reference conditions are pulse patterns identified from sampled audio signals that are consistent with patterns generated by three way call events. A sampling window and its associated profile are reset when reset conditions are detected. The profile is tagged as a potential three way call event when reference conditions are detected in a sample profile and no reset conditions are detected over the selected sampling interval (sampling window).

Abstract: A hybrid circuit for an electric operating and scanning interface is especially applicable to electronic recorders of electromechanical telephone stations. The hybrid circuit controls voltage and current parameters on different elements of the station in different situations. The hybrid circuit scans the condition of telephone station elements controlled by it, and programmably applies and receives current and voltage to and from the telephone station elements on which it acts.

Abstract: The invention provides a method for detecting a call progress tone on a pair of telephone conductors (the telephone line). The level of energy stored in an energy storage device is continually checked. When the level of energy of the energy storage device is below a threshold, the system couples the telephone line to an off-hook impedance. While coupled to the telephone line, the system charges the energy storage device using current drawn from the telephone line, and the system detects the presence of the call progress tone on the telephone line.

Abstract: A device capable of analyzing call progress event information from sensors connected to switched network circuits for the purpose of identifying the beginning and end of a telephone call. The invention also collects and analyzes all events occurring between the origination and termination of the call, providing a detailed description of both telephone user and network element actions. Abnormal events are identified during this analysis so that call quality can be determined. The invention can use information supplied by sensors that are intrusive or non intrusive to the switched network circuit.

Abstract: An automatic telephone recorder includes a computer equipped with a sound signal handling feature and a digital recording medium such as a disk drive. The sound signal handling feature includes an audio input, an audio output and programmable means for selectively effecting digital recording on the recording medium of an audio signal applied to the sound signal handling means input. A line transformer is connected between a telephone line pair and the sound signal handling means audio input. The sound signal handling means is programmed to periodically test the sound signal handling means audio input to determine if audio or a predetermined transient signal, indicating that the telephone line has gone from an on-hook condition to an off-hook condition, is present. If the audio or transient signal is present, the sound signal handling means is programmed to institute recording of any audio signal present at the sound signal handling means audio input for as long as such audio remains.

Abstract: For use with a telecommunications network, circuit for, and method of, detecting telephone line status and a telephone instrument embodying the same. The circuit includes: (1) a high-impedance amplifier, couplable to the telecommunications network, for providing DC voltage representing a differential line voltage between tip and ring conductors of the telecommunications network and (2) a pulse-generation circuit, coupled to an output of the amplifier, for generating a signal having first and second states, the signal assuming the second state for a period of time inversely proportional to the differential line voltage between the tip and ring conductors.

Abstract: A programmable timer circuit having ON and OFF volume control states; for controlling the volume of audible signals generated by a signal generator of a communication device at preprogrammed times is disclosed. The programmable timer circuit monitors a communication link for the presence of signals indicative of an incoming communication. When such signals are detected and the volume control state of the timer circuit is ON, the volume of the audible signals generated by the signal generator of the communication device is adjusted so that such audible signals are broadcast with a programmed loudness. Volume control of the audible signals generated by the signal generator does not prevent outgoing communications using the communication device. The fact that the volume of the audible signals generated by the signal generator is controlled, is not apparent to the calling party.

Abstract: A current detector for connection in series with a lead carrying current to be detected, has a first branch with a plurality of series connected diodes having an anode end and a cathode end for connection to the lead, a first optocoupler light emitting diode connected in series with a resistor, connected in parallel with the first branch, the optocoupler diode being connected in the same polarity direction as the diodes in the first branch, and a first further diode connected in parallel with the first branch, in opposite polarity direction as the diodes in the first branch.

Abstract: Disclosed is a three-way call detection system that uses digital signal processing to identify a third party connection. The disclosed invention operates by establishing a baseline ambient, or background, noise level, and detecting when the signal noise level drops below the ambient noise level. When the current signal noise level drops below the ambient noise level, the system assumes that a three-way conference call has been attempted by the called party.

Abstract: A circuit for sensing telephone line conditions, comprising: high impedance termination means across the telephone line; means for accumulating charge through the high impedance termination means; and electrical isolator means intermediate the means for accumulating charge and voltage detecting means.

Abstract: A ring generator circuit is included in a subscriber line interface circuit (SLIC) for providing a high voltage ring signal to a plurality of ringers at the same time. The ring generator circuit includes an adder and divider circuit connected to receive a DC offset and an AC reference signal from the SLIC. The adder and divider circuit outputs a fractionalized summed signal. That signal is supplied to a full-wave rectifier, which rectifies the fractionalized summed signal so that it is always a positive voltage. The rectified signal is supplied to a switch control circuit, a first amplifier and a second amplifier. The first and second amplifiers have the same gain, but with opposite signs or polarities. The outputs of the first and second amplifiers are sent to a commutating switch, which is controlled by an output of the switch control circuit.

Abstract: An off-hook telephone clearing system for a telephone line having at least two telephones connected thereto, comprising a howler tone detector connected to each telephone, and a telephone disconnect device having an input responsive to the howler tone detector for disconnecting a telephone being in off-hook condition from the telephone line. The clearing system may include a latching relay in the disconnect device. The latching relay may be mechanically coupled to the telephone hookswitch for delatching upon hang-up of the receiver.

Abstract: A technique for converting specific dial tone signals received from a telephone exchange, such as those which indicate a Message Waiting, into formats that are readily recognized by telephones or adjunct devices is disclosed. A dial tone converter device is connected between the telephone exchange providing the dial tone signalling and a message indicator device (such as an integrated telephone or message display adjunct device) which does not recognize dial tone signals. Alternately, the dial tone converter device is connected in parallel with the message indicator device. Under normal conditions, the message indicator device is connected to the telephone exchange in a way that all types of signalling (including dialing, voice, and FSK) can pass to and from the exchange. At specific intervals when neither the telephone nor any extensions at the subscriber's number are in use, the dial tone converter engages the line such that the dial tone signal is provided by the telephone exchange.

Abstract: A method and an apparatus for off-hook detection in a subscriber line interface circuit responds to a ringing signal which preferably lacks a DC component. A signal is generated that is a function of the line current through the subscriber line, and the time at which the signal exceeds a predetermined current value is measured. The measured time is compared with a predetermined time value, and an off-hook indication is produced in a processor depending on the comparison.

Abstract: A parallel-connected telephone use determining circuit using a microcomputer having therein an analog-to digital converter for determining an operation status of a parallel-connected telephone system connected to a telephone line. The circuit includes a rectifier for rectifying the variance of voltage applied through a telephone line, a voltage detector for detecting the variance of the voltage rectified from the rectifier, and a central processing unit (CPU) having an A/D converter incorporated therein for converting the voltage variance output from the voltage detector into a digital signal, for detecting the digital signal converted from the A/D converter and determining whether a parallel-connected telephone is in use.

Abstract: In addition to the conventional telephone portion, this telephone apparatus, coupled to a subscriber's (TEL) line coupled to an intercommunication (ICM) line, comprises a modulation circuit for modulating a data signal to have a frq band different from that used in the conventional telephone portion; a demodulation circuit for receiving/demodulating a modulated data signal having that frq band from another telephone apparatus connected to the same ICM line; and a control circuit for effecting communication with the TEL line and generating control data indicative of inhibition of using the TEL line and transmitting it as the data signal toward another telephone apparatus when the user responds a call or demodulation circuit does not receive modulated data signal indicative of inhibition of using the TEL line from another telephone apparatus. A busy tone may be supplied to the user when the user tries to use the TEL line if the control data is received by the demodulation circuit.

Abstract: In a ring trip circuit, a trip detecting circuit includes a low pass filter connected to opposite ends of a first resistor detecting an AC ringing signal, a diode bridge connected to an output of the low pass filter, a first series circuit connected to an output of the diode bridge and composed of a Zener diode and a light emitting diode of a first photocoupler, so that a phototransistor of the first photocoupler outputs a trip detection signal to a circuit which can be driven with a different power supply. A zero-crossing point detecting circuit has a second series circuit connected to the opposite ends of the first resistor and composed of a second resistor, a second Zener diode and a light emitting diode of a second photocoupler, so that a phototransistor of the second photocoupler outputs a zero-crossing point detection signal to a circuit which can be driven with a different power supply.

Abstract: A novel telephone system and method for activating a speakerphone in response to an incoming call based upon the presence or absence of a subscriber within a predetermined proximity zone. The telephone system has a ring detector for detecting a ring signal in response to an incoming call received via a central switching network, a proximity sensor for sensing the presence or absence of a subscriber within a predetermined proximity zone of the telephone system, and a control unit for automatically activating the speakerphone to establish communication with the caller when the subscriber is within the predetermined proximity zone and deactivating the speakerphone in response to detecting a busy tone generated by the central switching network when the call is terminated.

Abstract: An apparatus for detecting an off-hook condition on a telephone, the apparatus being connected to the tip line and the ring line of a telephone line, a first potential difference existing between the tip line and the ring line, the telephone line connected to the telephone having a handset. A circuit generates a difference signal representative of the absolute value of the first potential difference. A first comparator, responsive to the difference signal, generates a first indicator signal when the difference signal is above a first value corresponding to a maximum value of potential difference between the tip line and the ring line when the handset is off-hook. A second comparator, responsive to the difference signal, generates a second indicator signal when the difference signal is below a second value corresponding to a minimum value of potential difference between the tip line and the ring line when the handset is off-hook.

Abstract: A telecommunications device (10) includes an input terminal (12,13) for coupling to an input line of a telecommunications network. A detection arrangement (17, 18, 23) is coupled to the input terminal (12,13) for providing an output voltage in response to a voltage on the input line exceeding a threshold level. An output terminal (21) is coupled to receive the output voltage from the detection arrangement (17, 18). The detection arrangement (17, 18) includes a control arrangement (17) for setting the threshold level in response to a received control signal (19).

Abstract: The invention provides an apparatus for use with a subscriber line interface circuit in a telephone system, the subscriber line interface circuit being interposed between a telephone central office and a subscriber line, the subscriber line being coupled with a subscriber telephone device.