This circuit can be used in office or at home, if a telephone is wire-bound, ie., if a telephone is connected to number of extensions, this circuit will be a useful one for it.The working of the circuit is as follows: When someone is using the phone, the indicator light will be glowing to indicate that the directed line is in use, which informs temporarily not to use the telephone.
Indicator is connected on the phone lines in parallel. The D1 ~ D4 acts as a bridge, which provides power to the circuit. When someone uses the telephone, the telephone line voltage drops down to 12V and so the transistor T2 gets OFF, which in turn switch ON the transistor T1. So, the transistor T1 makes the LED to glow.