A key telephone system for use with at least two telephone lines that extend between a central office and two or more telephone stations. Each station is capable of responding to a manually-actuated hold select switch by establishing a hold condition on one of the lines and by connecting a line terminating...http://www.google.fr/patents/US4803718?utm_source=gb-gplus-shareBrevet US4803718 - Hold detecting and control circuit for a key telephone system
Hold detecting and control circuit for a key telephone system
