Abstract: A communication system for combining telephone service with hands-free talk-back intercom service is disclosed. The telephone service includes a telephone exchange network having an outside trunk, an internal trunk, and a device for switching the telephones within the system between either the outside trunk or the internal trunk in response to appropriate signals from the telephones. The intercom network generally includes a main distribution station associated with various telephones and transducers. Various embodiments of the system, having differing numbers of the above elements, are disclosed.

Abstract: A hands-free talk-back two-way intercommunication system is disclosed. The system includes a plurality of local stations connected to a main distribution station. Also connected to the main distribution station may be a direct station selector. At least one of the local stations includes a telephone and at least one of the other local stations includes a transducer element. The telephone and the transducer element may be connected in communication with each other through the main distribution station which includes a listen/talk channel controlling member, a call monitoring member and a station selecting member. The listen/talk channel controlling member includes apparatus for adjusting the balance between the listen and talk channels. The channel controlling member also includes apparatus for providing a notification signal to any transducer member which is addressed by the telephone.

Abstract: A safety circuit for protective operation at the interface between certain customer premises telephone installations and the external telephone distribution lines. The protection circuit includes a means for discriminating any voltages of sufficient magnitude having a frequency at or near 60 Hz., and upon such detection, a reactance means is responsive to open the connection between the premises telephone installation and the external telephone distribution line.