This telephone off hook indicator circuit will activate when the telephone receiver is taken off hook. The circuit is using an LED and a peizo buzzer, which will provide visual and sound indications. When the telephone is off hook the LED will start flashing and the peizo buzzer will provide beeping sound

The circuit is very useful and can be used for many telephone related tasks, for example it will play a very important role at the time when accidentally phone receiver is not putted correctly on the phone. It can also be used to indicate the phone is busy if a phone has many extensions.

The circuit is using two transistors one NE555 IC and other discrete components. The phone line has 48V DC when the phone is on-hook. But this voltage drops to around 10 volts when the phone is off-hook, when this voltage drops the transistor Q1 conduct due to which the transistor Q2 conduct and activate the 555 IC. When the 555 IC will become activated the LED will start flashing and the buzzer will provide beeping sound signals.
The frequency of flashing rate of the LED and beeps can be controlled by the 100K variable resistor. The operating voltage of the circuit is 6 to 12V DC. 

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Telephone Off Hook Indicator Circuit Diagram