Adjective
Describing the state of a telephone (or similar device) when it is available but not in use
(by extension) Describing the idle state of a subscriber or PBX user loop
Source: en.wiktionary.orgNormally, a PBX is connected via a T1 with robbed bit signaling to indicate on-hook or off-hook conditions and MF and DTMF tones to encode the destination number. Source: Internet
In a one-way trunk, the originating or A board sends a short for off-hook, and an open for on-hook, while the terminating or B board sends normal polarity or reverse polarity. Source: Internet
Note that if off-hook pertains to one state, on-hook pertains to the other. Source: Internet
On a trunk, on-hook and off-hook signals must pass in both directions. Source: Internet
The PBX does not offer dial tone to the calling extension: it only detects on-hook or off-hook status. Source: Internet
The supervision lamps light to alert the operator when the parties finish their conversation and go on-hook. Source: Internet