Abstract: A telephone headset apparatus for use with an existing cellular telephone. The headset apparatus is modularly connectable to the existing cellular telephone system, and no electrical or mechanical modification of the existing telephone system is required. A "handset primacy" feature of the headset apparatus ensures that normal handset operation of the cellular telephone system is unaffected by the installation of the headset apparatus. In addition, the handset may remain resting in its cradle even while the headset is operated. A slave unit connects with the cellular telephone's transceiver unit, and a headset control unit connects both the headset and handset to the slave unit. The headset control unit contains a pushbutton for selecting headset operation, as well as a volume control dial for controlling the volume of sound reproduced by the headset's earpiece.