The ring relay control unit of an FXS includes a main control unit for controlling relay interrupt command; a relay control device for generating a relay control signal in accordance with a relay interrupt command transferred from the main control unit, an SLIC for interfacing a call signal to a subscriber's phone, a ring signal generator for generating a ring signal transferred to the subscriber's phone, and transferring a synchronous signal, which generates a relay control signal, to the relay control device, and a relay for selectively transferring the call signal and the ring signal to the subscriber's phone from the SLIC and the ring signal generator.