In telephony

Telephony

In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....

, the term off-hook has the following meanings:

1. The condition that exists when a telephone
   Telephone
   The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
   
    or other user
   User (telecommunications)
   In telecommunications, a user is a person, organization, or other entity that employs the services provided by a telecommunication system, or by an information processing system, for transfer of information....
   
    instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece (i.e., receiver), which hung from its switchhook until the user wished to activate it. The weight of the receiver no longer depresses the spring-loaded switchhook, thereby connecting the instrument to the telephone line
   Telephone line
   A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
   
   .
2. One of two possible signaling states, such as tone or no tone and ground
   Ground (electricity)
   In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
   
    connection
   Electrical connection
   An electrical connection between discrete points allows the flow of electrons . A pair of connections is needed for a circuit.Between points with a low voltage difference, direct current can be controlled by a switch...
   
    versus battery connection. Note that if off-hook pertains to one state, on-hook
   On-hook
   In telephony, 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...
   
    pertains to the other.
3. The active state, i.e., a closed loop of a subscriber line or PBX user loop.
4. An operating state of a communications link in which data
   Data
   The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
   
    transmission
   Transmission (telecommunications)
   Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
   
    is enabled either for (a) voice or data communications or (b) network
   Telecommunications network
   A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
   
    signaling.

On an ordinary two-wire telephone line, off-hook status is communicated to the telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

 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

Permanent signal

Permanent Signal in US telephony jargon, or Permanent Loop in British usage, is a condition in which a POTS line is off-hook without connection for an extended period of time. This is indicated in modern switches by the silent termination after the off hook tone times out and the telephone...

.

The act of going off-hook is also referred to as seizing the line or channel.