offhook

Off-hook is a term used in telecommunications to describe the state of a telephone line or device when the handset is lifted or the user initiates a call. In this condition, the hook switch is actuated, closing a circuit and allowing current to flow on the line. The exchange or private branch exchange (PBX) detects this change and provides dial tone so the user can enter numbers. The line is considered seized for the duration of the call, preventing other users from using that channel.

Historically, off-hook derives from the mechanical hook switches in traditional telephones; the term persists in digital and IP networks as well. In analog PSTN systems, supervision circuitry monitors loop current to detect off-hook and then routes the call or provides dial tone. The on-hook state occurs when the handset rests in its cradle and the circuit is opened, returning the line to idle.

In modern systems such as VoIP, IP phones, and mobile devices, off-hook is signaled by the signaling protocol when a call is initiated or answered, and the media path is established. Off-hook status can be used by call-control software and computer-telephony integration (CTI) to indicate availability or to route calls. The concept remains central to dialing, dial tones, busy signals, and other call-progress tones, even as networks have moved from purely mechanical to digital signaling.