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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 ...

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