Abstract: A tone responsive disabling circuit for disabling prior to the transmission of data on a telephone transmission line a device which is connected to the line for improving voice transmission on the line. The disabling circuit operates in response to the transmission on the line of a tone signal whose frequency falls within a predetermined band width and which also has a predetermined amplitude and minimum duration. The disabling circuit processes the signal transmitted on the line to generate two control signals. One of the control signals is a function of the transmission line signal. The other is a function of the energy contained in the transmission line signal which lies in a predetermined frequency band which is not the tone frequency band. When the control signal which is a function of the transmission line signal is greater in amplitude than both a reference signal and the energy related control signal, a phase locked loop is enabled to capture the tone signal and disable the voice improving device.