Key telephone system

Abstract

A key telephone system comprises a plurality of key telephones and a main equipment connected to at least one set of central office (CO) lines and capable of connecting thereto the key telephones. The main equipment includes a dual tone multifrequency (DTMF) sending circuit common to the key telephones, and the respective key telephones are connected to the main equipment through four wiring cables capable of forming one both-way voice frequency talking circuit, two one-way voice frequency talking circuits and one both-way digital data transmitting circuit. A plurality of switching matrixes are included in the main equipment for selectively connecting the both-way voice frequency talking circuit or one-way voice frequency talking circuit to the CO lines, the two one-way voice frequency talking circuits to the both-way voice frequency talking circuit or to one of the one-way voice frequency talking circuits in the other key telephone and also connecting the digital data transmitting circuit through the DTMF sending circuit to the CO lines, whereby respective functions of originating and terminating CO line calls, hand-free talk-back and off-hook signalling for intercom call, speaker monitoring function during CO line talking and the like are performable.