This simple circuit tells you about the status of phone such as Line OK, Dialing and Call attended. It also has a lock facility to block outgoing calls keeping the incoming calls as usual. This prevents misuse of telephone. The circuit uses only a few components to do all these jobs.
The circuit is directly connected to the telephone lines and no power supply is needed. In the On Hook state, the telephone lines have around 48 volts which reduces to 12 volts in Off Hook state. More over the line polarity changes during dialing and two way speech. Capacitors C1 reduce the line voltage to a safer level for the operation of LEDs. When the phone is Off Hook, Red LED lights to indicate that the lines are OK. When a call is dialed, Red LED blinks to indicate the dialing status. When the remote person attends the call, Green LED lights indicating that the call is attended.
A simple Toggle switch is provided as Lock. It can be replaced with an electrical lock with key. When the switch is in Off position, outgoing calls will be blocked but the telephone receives incoming calls as usual.