2 via the positive feedback circuit 50 at this time the capacitor c 22 has a function for shortening the turn on time of the transistor q 2 when a dial input takes place in an off hook state the microcomputer not shown controls the potential of the terminal hc so as to correspond to the dial input as a result the transistor q 1 sends a dial pulse signal on the other hand when on hook the potential of the terminal hc is controlled to be a low potential as a result of control by the microcomputer not shown as a result the base potential of transistor q 2 falls and the transistor q 2 therefore turns off in doing so the base potential of transistor q 1 falls and the transistor q 1 therefore turns off next a description is given of the operation when an overvoltage is applied to the subscriber lines l 1 l 2 when an overvoltage is applied to the subscriber lines l 1 l 2 a large voltage drop occurs at the overcurrent detection circuit 40 when this voltage drop exceeds the zener voltage the zener diode d 20 enters a breakdown state the breaker circuit 70 then causes the base potential of the transistor q 2 to fall at this time a reverse bias voltage is applied across the base terminal b 2 and emitter terminal e 2 of the transistor q 2 the transistor q 2 therefore turns off the instant within two microseconds the overvoltage is applied across the subscriber.