PURPOSE: To reduce the number of signal lines for identifying individual telephone sets and, at the same time, to always monitor disconnection faults of the signal lines by providing identifying signals having different frequencies by telephone sets, an answer signal which is commonly used for all telephone sets and has a frequency which is different from those of the identifying signals, and off-hook signal.

CONSTITUTION: This system for identifying individual telephone sets is constituted of an individual telephone set identifying device 1, a two-wire signal line 2, connected to the device 1 and plural telephone sets (3-1)-(3-n) connected to the line 2 in a branched state. The device 1 sends an identifying signal Si to the signal line 2 and, when the device l receives an answer signal R* within prescribed time, sends another identifying signal Si+1 to the line 2. The device l successively sends identifying signals S1-Sn for all telephone sets by repeating the above-mentioned operations and, when the answer signal R* is not received, discriminates that a fault occurs on the signal line 2 beyond the telephone set which has responded to the identifying signal. In addition, by sending the identifying signal of a certain telephone set after starting the reception of an off-hook signal R#, the device l discriminates that the telephone set is in an off-hook state when the off-hook signal is stopped.