Abstract: An interface device for mating transducer-based telephone handsets to a telephone, and particularly transducer-based telephone handsets having different electrical input/output requirements comprises: a current source device for providing current flow through a transducer of the telephone handset; a device for detecting the current flow through the transducer present in the telephone handset and generating a handset type signal according to the amount of current flow detected; and, an adjusting device for adjusting the electrical characteristics of audio signals to be transmitted according to the generated handset type signal. The interface device further includes a control device responsive to the handset type signal for maintaining the current source device in an ON state to provide a predetermined amount of current flow to the telephone handset, or an OFF state substantially reducing current flow to the telephone handset.