Federal Standard 1037C MIL-STD-188 On an ordinary two-wire telephone line, off-hook status is communicated to the telephone exchange by a resistance short across the pair. Source: Internet
Normally, 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
In Wink start trunks, the exchange at the originating end sends an off-hook to alert to a call. 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