Abstract: A telephone having a semiconductor line switch TR1 is protected from excess line voltage or current by the use of current foldback techniques. A current sensing resistor R6, in series with the line switch provides a measure of the line current and this operates a bypass transistor TR4 which reduces the drive current for the line switch TR1 causing it to come out of saturation and begin to block line current. A voltage sensing resistor R7 provides a measure of the voltage across TR1 and this further increases the bypass current through TR4. This ensures that the voltage and current applied to TR1 remain within permitted limits.