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The state of a telephone line that allows dialing and transmission but prohibits incoming calls from being answered. The phone is off-hook when the handset is lifted off the base of a stationary phone or when Talk is pressed on a portable phone.

The term stems from the days when the handset rested on an actual hook. When it was removed, a spring caused contacts to press together, closing the circuit from the telephone to the switchboard in the central office (CO). When a handset is placed back on the base, it is said to be "on-hook," and the phone can receive a call. See central office.

There Really Was a Hook
The hook on this Kellogg telephone (circa 1899) was attached to a spring. When the handset was removed, the contacts inside completed the circuit between the telephone and the central office.

central office

A local telephone company switching center. There are two types of central offices (COs). The first is called an "end office" (EO) or "local exchange" (LE) and connects directly to the outside plant, which is the feeder and distribution system to homes and offices. The end office (often called a "Class 5 office") provides customer services such as call waiting and call forwarding.

Tandem/Toll Office
The second type is the tandem office (also toll office or tandem/toll office), which is a central office that does not connect directly to the customer. Toll call record generation and accounting used to be handled in the tandem offices. Today, the billing is mostly done in the end offices. There are more than 25,000 central offices in the U.S. See LEC, IXC and LATA.

Central Offices
End offices provide the local loops directly to customers, while tandem offices carry traffic between end offices. The PSTN is made up of local exchange carriers (LECs) and interexchange carriers (IXCs) that are governed by LATA boundaries.

High-Tech Switching in the 19th Century
This "central office" was used to switch phone calls between all the lawyers in Richmond, Virginia in 1882. (Image courtesy of AT&T.)

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