What is "off-hook"

In telephony, the term off-hook has the following meanings:

1. The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating.
2. One of two possible signaling states, such as tone or no tone and ground connection versus battery connection. Note that if off-hook pertains to one state, on-hook pertains to the other.
3. The active state (i.e., a closed loop ( short circuit between the wires) of a subscriber line or PBX user loop)
4. An operating state of a communications link in which data transmission is enabled either for (a) voice or data communications or (b) network signaling.

On an ordinary two-wire telephone line, off-hook status is communicated to the telephone exchange by a resistance short across the pair. When an off-hook condition persists without dialing, for example because the handset has fallen off or the cable has been flooded, it is treated as a permanent loop or permanent signal.

The act of going off-hook is also referred to as seizing the line or channel.