Editing On- and off-hook

{{Short description|A telephone respectively off- and on-line}}
{{Use American English|date=September 2024}}

In [[telephony]], '''on-hook''' and '''off-hook''' are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing the line''. ''Off-hook'' originally referred to the condition that prevailed when telephones had a separate earpiece (''receiver''), which hung from its [[switchhook]] until the user initiated a telephone call by removing it. When off hook the weight of the receiver no longer depresses the spring-loaded switchhook, thereby connecting the instrument to the [[telephone line]].

==Off-hook==
[[File:Trimphone, Off Hook.JPG|thumb|  Off hook telephone. ]]
The term '''off-hook''' has the following meanings:
* The condition that exists when a [[telephone]] or other [[User (telecommunications)|user]] instrument is in use, i.e., during [[Dialling (telephony)|dialing]] or communicating.
* A general description of one of two possible [[Signaling (telecommunications)|signaling]] states at an interface between telecommunications systems,<ref>{{cite web
 | url = https://archive.org/stream/bitsavers_westernEleEngineeringandOperationsintheBellSystem2_49741719/Engineering_and_Operations_in_the_Bell_System_2ed_1984
 | title = Engineering and Operations in the Bell System, 2nd Ed
 | last = Rey
 | first = R. F.
 | date = 1984
 | publisher = Bell Telephone Laboratories
 | access-date = 19 March 2017
 | page = 267}}</ref> such as tone or no tone and [[Ground (electricity)|ground]] [[electrical connection|connection]] versus battery connection. Note that if off-hook pertains to one state, [[on-hook]] pertains to the other.
* The active state (i.e., a closed loop ([[short circuit]] between the wires) of a [[subscriber line]] or [[Private branch exchange|PBX]] user loop)
* An operating state of a communications link in which [[transmission (telecommunications)|data transmission]] is enabled either for (a) voice or data communications or (b) [[telecommunications network|network]] signaling.<ref>[[Federal Standard 1037C]]</ref><ref>[[MIL-STD-188]]</ref>

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.

==On-hook==
[[File:Original Trimphone, On Hook.JPG|thumb| On hook telephone handset]]
The term '''on-hook''' has the following meanings:  
* The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook. The weight of the receiver depresses the spring-loaded switchhook thereby disconnecting the idle instrument (except its bell) from the telephone line.
* One of two possible signaling states, such as [[SF tone|tone]] or no tone, or [[Ground (electricity)|ground]] connection versus battery connection. Note: if on-hook pertains to one state, off-hook pertains to the other.
* The idle state, i.e., an open loop of a [[subscriber line]] or [[Private branch exchange|PBX]] user loop. 
* An operating state of a [[telecommunication circuit]] in which transmission is disabled and a high [[Electrical impedance|impedance]], or "open [[Electronic circuit|circuit]]", is presented to the link by the [[end instrument]](s). Note: during the on-hook condition, the link is responsive to [[Ringing (telephony)|ringing signals]].

The act of ''going on-hook'' is also referred to as ''releasing the line'' or ''channel'', and may initiate the process of [[clearing (telecommunications)|clearing]].

== See also == 
* [[Disconnect supervision]]
* [[Hook flash]]
* [[Line signaling]]
* [[Off-hook tone]]

== References ==
{{FS1037C MS188}}
{{reflist}}

[[Category:Telephony signals]]