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. It is sometimes confused with other concepts of telephone geography, such as NPA or area code.

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