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.

Abstract: A key telephone system comprises a main equipment connected to at least one set of central office (CO) lines and capable of connecting thereto a plurality of 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 six wiring cables forming a pair of two-way voice frequency talking circuits and one two-way data transmitting circuit. A plurality of switching matrixes are included in the main equipment for selectively connecting the CO lines and the talking circuits, and also connecting the data transmitting circuit through the DTMF sending circuit to the CO lines and one of the talking circuits in the key telephone, whereby respective functions of hand-free talk-back and off-hook signalling during intercom talking, and speaker monitoring function during CO line talking are performable.