PURPOSE: To relieve the control load of a main control section by executing changeover control, speech control and data control by an output signal from monitor circuit section generated when a digital information channel signal from an extension terminal equipment reaches a prescribed value.

CONSTITUTION: The digital information channel signal is fed to a separation circuit section 36, in which the signal is separated into a synchronization clock signal F, a voice channel signal B1, a data channel signal B2 and a control section channel signal D. The signals F, B1 are accepted by the shift register of a monitor circuit 37, the B1 signal is latched by the signal and registereed while being shifted sequentially. It is confirmed that an exension terminal equipment is required talking through the shift to eliminate mistakes and undesired speech, When the digital information channel pattern signal becomes a registered value, a signal H is generated. Arithmtic processing is executed by an H signal and the talking between an extension terminal equipment and a trunk line terminal equipment is executed. Only the signal required for speech or the like is selected from the D signals by the talikgn and the control load is remarkably relieved.