Prefix

Understanding how telephone numbers are assigned is very important when you are designing or implementing a telephone sample. Telephone numbers in the United States, Canada, and the Caribbean consist of 10 digits divided into three components. The first three digits are the area code or Numbering Plan Area (NPA). Area codes usually cover large geographic areas. The next three digits represent smaller geographic areas within an area code and are referred to as the prefix (NXX). The term prefix is also used to refer to the first three digits of the seven-digit local numbers. When the NXX is combined with the final four digits of a phone number, the result is a unique seven-digit local number that is associated with a unique end user within an NXX. In most areas of the United States, local seven-digit dialing of landlines is the norm. Within these areas, the area code component of a 10-digit number need not be dialed, and calls are switched using only the prefix. Although area codes are unique, prefixes are unique only within an area code. Thus, in making a long-distance telephone call or a call to a different area code, the area code is required in order to define a unique prefix. Not all prefixes are available for assignment of local numbers. Some are reserved for other uses, such as directory assistance (555) and special services such as 911 and 411.

Although the terms prefix, wire center, rate center, central office, and exchange are frequently used interchangeably, there are distinctions. The wire center, rate center, or central office is the building containing the telephone equipment and switches where individual telephone lines are connected and through which calls to and from individual local numbers are routed. The term exchange usually refers to the geographic area served by a particular rate center or wire center. Thus an exchange can be serviced by multiple prefixes (NXX codes). In some areas of the United States, these prefixes might belong to different area codes.

Each prefix contains 10,000 possible local numbers (0000–9999). Within prefixes, local telephone [Page 608]companies assign four-digit local telephone numbers. During the past 20 years the demand for telephone numbers has dramatically increased, driven by technological advances and governmental regulations relating to cell phones, pagers, ATMs, faxes, computers, Internet access, and local number portability. In order to meet this demand, the North American Numbering Plan (NANP) was modified to allow for what became known as interchangeable area codes. Originally an area code could not be the same as a prefix number, and a prefix number could not be the same as an area code number. Starting in 1973, prefix codes could be any number in the format NXX where N is a number between 2 and 9 and X a number between 0 and 9. In 1995, area codes were allowed to be any number in the same format (NXX).

Interchangeable area codes are also allowed for area code overlays, where two or more different area codes can service the same geography. In some areas, the borders for new area codes have been drawn to conform to legal geographic boundaries, such as city or county lines. Instead of moving entire prefixes to a new area code, prefixes were “partitioned” into two or three pieces across two or three different area codes. Today, an exchange can be serviced by prefixes in multiple area codes, in which case 10-digit dialing is mandatory. For this reason, many people refer to the NPA-NXX as the prefix.

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