PURPOSE: To quickly respond when an incoming arrives at one of plural incoming telephone system terminals connected to an accommodated line by specifying the incoming telephone system terminal concerned by the incoming request information of the incoming.

CONSTITUTION: On the off-hook of a telephone set A, the CPU 6 of a private branch exchange 1 stores previously the extension number of the telephone set A of a calling party, for instance '20', in a RAM 820 in order to use it for connecting a channel. Then, it stores the extension number '21' and an incoming discrimination number '3' from the telephone set A detected by the signal supervising part 540 of an extension interface part 5 in the RAM 820. Then, it investigates the idle state of the extension 3 corresponding to the extension number, and if it is not busy, it decides the existence of an incoming number according to the RAM 820, and it not exist, it outputs a standard call signal to the extension 3. On the other hand, if the incoming number exists, it outputs a corresponding call signal to the extension 3. Next, on the called response of the telephone set F, it executes a communication connect instruction to the calling extension stored in the RAM 820, and a communicating state between the telephone sets A and F is started.