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In telephony, the term off-hook has the following meanings:
- The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating.
- 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.
- The active state (i.e., a closed loop (short circuit between the wires) of a subscriber line or PBX user loop)
- 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.
Off-hook originally referred to the condition that prevailed when telephones had a separate earpiece (i.e., receiver), which hung from its switchhook until the user wished to activate it. The weight of the receiver no longer depresses the spring-loaded switchhook, thereby connecting the instrument to the telephone line.
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