switch hook

A switch hook is a component in a telephone that connects the instrument to the electrical current supplied through the local loop. In older telephones, the receiver was placed on a hook that would open or close a metal contact. When the phone was "on hook," the connection was broken. Lifting the handset ("off hook") restored the connection, allowing current to flow and enabling the user to make a call. Modern electronic instruments may use transistor relays instead of mechanical contacts. The lifting of the handset signals the switching office to transmit a dial tone.12