PURPOSE: To realize the monitor of each slave station, a call between slave stations and a simultaneous command to each slave station, by providing a line transformer circuit having input terminals corresponding to plural slave stations to a master station.

CONSTITUTION: A key S1 is operated to connect a station A and a line transformer circuit 13 during a call with the designated station A. Then a call is carried out by a telephone set 11 via bilateral amplifier circuit 12. When a call is carried out between designated slave stations, the key S1 is operated in response to the output of a monitor amplifier circuit 14 to connect the station A and the circuit 13. Then a key MU2 and an MU contact are operated to connect a station B and the circuit 12. Thus a bilateral call is carried out between stations A and B, and the contents of this call are monitored by the circuit 14. When a simultaneous calling is given to all slave stations from a master station, contents ALL1∼ALLn are actuated to connect all slave stations and the circuit 13. Then a simultaneous command is delivered to each slave station. In addition, it is always possible to monitor all slave stations with operation of a prescribed key.