Hold control for a key telephone system

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. A line on which a call is in progress may be placed in a hold condition by operating the hold select button which thereafter enables the telephone set to be coupled to another line, if desired, for receiving or placing another call. A conference call can be established by simultaneously operating the two or more line select switches which in turn cause the selected lines to be jointly connected to the telephone set. The indicator lights on each control unit respond to standard central office supervisory signals which appear on the telephone lines and which represent on-hook (idle), ringing and off-hook (busy) line conditions, and also respond to a special hold indicating signal which is applied to the line by the control unit during the condition. The control units of the various telephone sets operate solely in conjunction with the standard telephone lines that extend from the central office and no additional interphone wiring is required and no central control unit is needed.