PURPOSE: To attain talking between digital telephone sets connected to a same bus without charge imposition by setting up a call not through a network in ISDN compatible digital telephone sets and to transfer a call set up from the network to digital telephone sets connected to a same bus without use of the additional service of the network.

CONSTITUTION: The telephone set is provided with a communication interface circuit 3 on a same bus composing an individual communication interface between digital telephone sets connected to the same bus, a control signal detection circuit 4 detecting a control signal on the communication of the same bus and a control signal transmission circuit 5 sending the control signal for the communication on the same bus in addition to components for a conventional digital telephone set.