PURPOSE: To realize the extension communication between plural terminal equipments connected to a subscriber's wiring by adding a signal reception function on an incoming signal line and a signal sending function to an outgoing signal line in a 4-wire form subscriber's respectively.

CONSTITUTION: A signal is sent to an incoming signal line (T line) from terminal equipments TE(#1), TE(#2) when 2-calls of user bus are used for a conventional external communication in the 4-wire bus form subscriber's wiring (user bus) forming the ISDN user network interface, the signal is sent from a network terminator NT1 to the subscriber line and the signal from the subscriber line is received by an outgoing signal line (R line) via the NT1. In applying the extension communication between the TE(#1) and TE(#2), the TE(#1) sends a signal to the T line and the signal of the T line is received by the TE(#2) as it is, and the signal is sent to the R line by the TE(#2) and it is received by the TE(#1) as it is.