Off-hook
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In telecommunication, the term off-hook has the following meanings:
1. In telephony, the condition that exists when an operational telephone instrument or other user instrument is in use, i.e., during dialing or communicating.
Note: Off-hook originally referred to the condition that prevailed when the separate earpiece, i.e., receiver, was removed from its switchhook, which extended from a vertical post that also supported the microphone, and which connected the instrument to the line when not depressed by the weight of the receiver.
2. One of two possible signaling states, such as tone or no tone and ground connection versus battery connection.
Note: If off-hook pertains to one state, on-hook pertains to the other.
3. The active state, i.e., closed loop, of a subscriber 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.
Source: from Federal Standard 1037C and from MIL-STD-188