Abstract

Standard four prong, plug-in telephone sets are modified to enable intercommunication among the sets on the subscriber's premises. In a subscriber telephone system including a single central office circuit, only two of the standard four prongs and associated receptacle sockets are used for establishing connection to the central office circuit. In the disclosed system, a local intercom circuit is added to the premises and connects to the remaining two sockets in each receptacle so that each set that is plugged in is also connected to the intercom circuit. The sets are modified to include a line selector switch which enables the talk circuits therein to be switched back and forth between the central office circuit and the intercom circuit. An intercom call signal generator and detector is also added to each set for intercom call signalling over the intercom circuit. In a subscriber telephone system that includes two separate central office circuits, the system is adapted to enable one of the two circuits to be used for intercom calls as well as central office calls. Switching back and forth between an intercom mode and a central office mode on that one circuit is accomplished by a suitable command such as a hook flash. The two central office circuit system also allows the standard four prong plug-in arrangement to be retained for the telephone sets. Calls may also be placed on hold and conference connections made in the two central office system upon the generation of appropriate hook flash commands.