Key telephone system having interstation signalling during hold condition

In a key telephone system of the type having a plurality of control units, one for each of a plurality of phone stations, for selectively connecting at least first and second telephone lines to a phone set at each such station, and in which each control unit is of the type having first and second line condition sensing means and associated indicator means for sensing and indicating conditions of on-hook, off-hook and hold on each of the first and second telephone lines, and having first and second hold control means including a hold signal generator for selectively applying a hold control signal to said first and second lines, respectively, in response to a manually operable hold select switch to thereby establish a hold condition on either or both of the lines during which hold condition the associated line is disconnected from the telephone set, the improvement comprising:

• interstation signalling means associated with a first of said control units for altering the hold control signal generated by said first control unit so as to apply an altered hold control signal to one of said lines at a first of the phone stations;
• interstation signal detection means associated with a second of said control units for detecting said altered hold control signal at a second of the phone stations;
• and
• interstation signalling device means disposed at the second station and being responsive to said detection means for indicating at the second station that said altered hold control signal has been received by said second control unit.