Abstract: The invention relates to a network station (1) comprising a selector switch (4) for coupling a switching center (2) to a POTS or ISDN telephone, and a detection circuit (3). After the detection circuit (3) has been made operative or after the mode change from the POTS mode to the ISDN mode, the detection circuit (3) controls the selector switch (4) such that this switch goes to the ISDN mode and thus couples the switching center (2) to an ISDN network terminator (11). When the ISDN mode is changed into the POTS mode, the selector switch (4) is actuated by the detection circuit (3) so that the switch couples the switching center (2) to the POTS telephone.

Abstract: The invention relates to a telephone connection box for an ISDN basic access. The telephone connection box comprises an electronic circuit of a network terminating device.

Abstract: The invention relates to a communication system comprising a station (2) for charge metering of a connection point. This station comprises a monitoring circuit (7) for detecting a request for a connection set-up from the connection point (3), either for establishing the absence of a message after a given period of time, or for evaluating the contents of a message that has arrived from the connection point (3) after the request for a connection set-up has arrived, and for rejecting the request for a connection set-up if invalid parameters of the connection point (3) are established.

Abstract: A code converter for processing switching information signals, which comprises a protection circuit for limiting a current flowing in a telephone exchange wire. The protection circuit includes an evaluation circuit, which causes a switch arranged in series with the telephone exchange wire to open if the current, or a signal derived from the current, exceeds a threshold value. In an embodiment of the invention the current is also limited if the current assumes intolerably high values only for a brief period of time and no opening of the switch is effected.