Abstract: To convert a multiple line, multiple telephone set installation into a key telephone system, a control unit is provided at each telephone set. The control unit includes a regulated power supply, a control circuit, pushbutton line select and hold switches, and indicator lights packaged in a compact assembly which can be readily attached to the base of a standard telephone set or mounted separately. Electrically controlled switching devices within each control circuit respond to the pushbutton line select switches to couple the telephone set to a selected one of the lines and automatically disconnect the set from the line when the call has been completed. An incoming ringing signal on any one of the telephone lines is detected by the control circuit and the ringing signal is automatically applied to the bell of the telephone set.