On-hook
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In telecommunication, the term on-hook has the following meanings:
1. In telephony, the condition that exists when an operational telephone, or other user instrument, is not in use.
Note: On-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a hook that extended from a vertical post that supported the microphone also. The hook was mechanically connected to a switch that automatically disconnected the idle telephone from the network.
2. One of two possible signaling states, such as tone or no tone, or ground connection versus battery connection.
Note: If on-hook pertains to one state, off-hook pertains to the other.
3. The idle state, i.e., open loop, of a subscriber or PBX user loop.
4. An operating state of a communications link in which data transmission is disabled and a high-impedance, i.e., open circuit, is presented to the link by the end instrument(s).
Note: During the on-hook condition, the link is responsive to ringing signals.
Source: from Federal Standard 1037C and from MIL-STD-188