On- and off-hook

  (Redirected from Off-hook)

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 telephone.

The term off-hook has the following meanings:

  • The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating.
  • A general description of one of two possible signaling states at an interface between telecommunications systems,[1] such as tone or no tone and ground 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 PBX user loop)
  • An operating state of a communications link in which data transmission is enabled either for (a) voice or data communications or (b) network signaling.[2][3]

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 telephone handset

The term on-hook has the following meanings:

The act of going on-hook is also referred to as releasing the line or channel, and may initiate the process of clearing.

See alsoEdit


  This article incorporates public domain material from the General Services Administration document: "Federal Standard 1037C". (in support of MIL-STD-188)

  1. ^ Rey, R. F. (1984). "Engineering and Operations in the Bell System, 2nd Ed". Bell Telephone Laboratories. p. 267. Retrieved 19 March 2017.
  2. ^ Federal Standard 1037C
  3. ^ MIL-STD-188