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Phone Exchanges


Info on phone number Exchanges

A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems also human operators that interconnect (switch) telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers.

In the public telecommunication networks a telephone exchange is located in a central office (CO), typically a building used to house the inside plant equipment of potentially several telephone exchanges, each serving a certain geographical exchange area. Central office locations are often identified in North America as wire centers, designating a facility from which a telephone obtains dial tone. For business and billing purposes, telephony carriers also define rate centers, which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements.

In the 1940's, in the United States and Canada, the Bell System established a uniform system of identifying each telephone exchange with a three-digit exchange code, or central office code, that was used as a prefix to subscriber telephone numbers. All exchanges within a larger region, typically aggregated by state, were assigned a common area code. With the development of international and transoceanic telephone trunks, especially driven by direct customer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries in the mid-20th century.

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