A ringer tone signal is attenuated, during a state in which a call is in progress, below the ringer tone signal level that occurs when no call is in progress, when a PC terminal is utilized as a telephone. A voice call system includes a handset unit for making a call, and a hook switch detection unit for detecting an on-hook/off-hook state of the handset unit. The PC terminal includes a sound source interface unit connected to a sound source, a fixed storage device for storing setting data indicative of a sound source for making a voice call, and a communication control unit responsive to a ringer tone generating operation command issued by the voice exchanger for determining whether or not a call is in progress from an on-hook/off-hook state detected by the hook switch detection unit.