Off-hook

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

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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