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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 .
An off-hook condition represents a circuit that is in use, e.g., when a telephone call is in progress. An on-hook condition represents an idle circuit, i.e. no telephone call is in progress. A central office originally was a primary exchange in a city with other exchanges service parts of the area.
Some central office switches in the United States, notably older GTD-5 EAX systems, utilize a single frequency tone, 480 Hz, known as High Tone for this purpose. In either case, the tone is substantially louder than any other signal transmitted over a copper POTS circuit; loud enough to be heard across a room from an unused off-hook telephone.
A dial tone (dialling tone in the UK) is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is working and is ready to initiate a telephone call. The tone stops when the first dialed digit is recognized.
Line signaling is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook (answer supervision and disconnect supervision, together referred to as supervision), ringing, and hook flash. Register signaling is concerned with conveying addressing information, such as the calling and/or called telephone ...
Many telephone service providers attempt to achieve dial-tone availability more than 99.999% of the time the telephone is taken off-hook. This is an often cited benchmark in marketing and systems-engineering comparisons, called the " five nines " reliability standard.
Line signaling is responsible for off-hook, ringing signal, answer, ground start, on-hook unidirectional supervision messaging in each direction from calling party to called party and vice versa. After an off-hook, line signaling initiates register signaling to accomplish the exchange of telephone numbers of called party and in more modern line ...
Operator assistance refers to service provided by a telephone operator to the calling party of a telephone call.This can include telephone calls made from pay phones, calls placed station-to-station, person-to-person, or collect, third-number calls, calls billed to credit cards, and certain international calls which cannot be dialed directly.