Abstract: An automated prefix dialing system is disclosed to allow a subscriber/user to automatically insert a predetermined prefix dialing code in certain outgoing telephone call situations. The system is electrically disposed between the telephone company's central office circuit and the subscriber's circuit. The system preferably comprises six sub-circuits: DC power path circuit (off-hook sensor circuit), shadow-ring detector circuit, isolation circuit, DTMF receiver circuit, DTMF generator circuit, and micro controller circuit. When not in use, no power is used by the system. The DC power path circuit, provides power received from the central office line to the various circuits and components of the system when the subscriber's phone is taken off-hook. Off-hook sensor circuitry detects when the telephone has been lifted either in response to an incoming call or to initiate an outgoing call.

Abstract: A circuit for checking a plurality of impedances having a common connection point as for use in an emergency medical ratio, where the impedance of each ECG electrode contact is automatically and sequentially checked during the initial calibration transmission, using the AC non-polarizing signal which is also used as the ECG subcarrier for the medical data telemetry. The ECG telemetry calibration signal is also processed to provide a sequential switching control. Poor electrode contacts are indicated visually to the paramedic in charge and thus can be corrected before ECG telemetry begins.