Abstract: A telephone set usable as an acoustic coupler comprising, a transmitter for generating an acoustic signal in response to an electric signal applied thereto, a receiver for generating an electric signal in response to an acoustic signal thereto, a telephone unit for performing telephone communication, a switch for switching over the operation mode of the telephone set, a switching controller connected to the transmitter, receiver and telephone unit for changing the operation mode of the telephone set, in response to the switching state of the switch, display connected to the switching controller for indicating the current operation mode of the telephone set, first connecting terminal connected to the telephone unit for connecting with an external device, second connecting terminal connected to the switching controller for connecting with an external device, and a belt attached to the telephone set for attaching the transmitter and the receiver to a hand set of another telephone set in alignment therewith, the