Central office line interfacing circuit

1. Technical Field A central office line interface circuit commonly detects a status of a ring signal and a hook switch. The central office line interface circuit includes a hook switch connected to a telephone line, a ring signal path connected in parallel to the hook switch, and a detection circuit connected between a controller of the exchange and a node of the hook switch and the ring signal path, so as to detect a ring signal in an incoming call mode and to detect a hook off status in an outgoing call mode.