PURPOSE: To call a key telephone set individually during an extension call by inverting and holding the polarity of a signal line, and by sending a correspond- ing individual call control signal only during the input of a key dialing signal by putting another key telephone set in an operation state.

CONSTITUTION: Key telephone sets TEL1∼TEL3 are connected to a master equipment ME through a couple of call lines TL and a couple of signal lines SL. Keys for key dialing are allotted in advance to the respective telephone sets, an extension call button is operated to hold the state setting circuit CS of an individual call control part ICCONT in readiness, and the relay RL of the circuit CS is operated through previous keying operation ato invert the polarity of the signal lines SL of the device ME. Only the individual call reception part ICR of a telephone set passed through a BPF for the tone frequency of a calling key operates to sends an output from a speaker, thereby calling a called telephone set. When a corresponding push button is pressed to call another telephone set during an extension call, the polarity of the call lines TL is inverted, so a diode D1 turns off while a D2 turns on to operate only the called ICR for calling the telephone set, thus performing a three-party call.