Key telephone system with a common push-button sender system

Claim 1

A key telephone system comprising a plurality of key telephone sets having at least a push-button dial and a shift register connected to said push-button dial, and a common unit connected to each telephone set via a signal line and to the shift register of each telephone set via a control line, said common unit including a common control, a line connection circuit connected to said common control and responsive to control signals on said control line, and a common push-button sender connected to said common control for sending dial tone and a DTMF signal according to the common control in response to a control signal applied to said control line by said shift register, characterized in that said common unit has at least a register link and a central office line link, said register link being connected to both a calling subscriber and said push-button sender so that said calling subscriber can hear said two-frequency signal while the calling subscriber selects one of the central office lines, said central office line link being connected to both the calling subscriber and the push-button sender so that the DTMF signal according to the dialed number by the calling subscriber is sent to both the calling subscriber and the office line.