PURPOSE: To recognize whether or not communication by a terminal equipment is enabled in advance without any operation of a terminal equipment by an operator by allowing the terminal equipment to inform whether or not the line connecting to the terminal equipment is available.

CONSTITUTION: An ISDN bus 21 includes 2 B channels, one D channel and an E bit in a control signal for contention control. B channel control circuit 22 connects the digital voice information of a communication control circuit 15 to the B channel. D channel control circuit 23 applies outgoing/incoming call control by using the D channel. In a call signal monitoring circuit 24 on the bus monitors, a signal on the D channel sent from a network terminator NT 31 to each terminal equipment is monitored the D channel and a signal on the D channel sent from a function to inform the result to CPU 11 and each terminal equipment is monitored by E bit to have the function informing the result to CPU 11. A channel control circuit 25 converts analog information generated in a handset 20 into digital information.