Patent US 4,092,597

A two-tone decoder decodes simultaneously at least two calling tones each having different frequencies coupled from a calling station to a called station and produces a first control signal upon cessation of the two calling tones. A timer responding to the first control signals produces a second control signal in the form of a pulse having a predetermined duration. A relay is activated by the second control signal and couples a confirmation tone to the calling station from a confirmation tone generator in the called station for the predetermined duration of the second control signal.