Telephone Exchange Building

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make telephone calls "work" in the sense of making connections and relaying the speech information.

The term exchange can also be used to refer to an area served by a particular switch (typically known as a wire center in the US telecommunications industry). It is sometimes confused with other concepts of telephone geography, such as NPA or area code. More narrowly, in some areas it can refer to the first three digits of the local number. In the three-digit sense of the word, other obsolete Bell System terms include office code and NXX. In the United States, the word exchange can also have the legal meaning of a local access and transport area under the Modification of Final Judgment (MFJ).