This circuit is connected across the phone line, and the pushbutton switch is placed close to the phone. When the pushbutton is pressed, the red LED lights up. The circuit is polarity sensitive so if the LED does not light, then the connections may have to be reversed. Once the LED is lit, the phone can be hung up, and then the other extension may be picked up. As soon as either phone is picked up, the LED will go out and conversation can resume.