There are 680 usable area codes in the United States, of which 215 are currently in use. Each area code has 7,920,000 telephone numbers (out of a possible 10,000,000) available within it. Some numbers, such as those that would start with 0,1 or 911, are unavailable for use. Others, like 555, which is used as a prefix for fake phone numbers in movies and on TV, are reserved for special use.
The numbers within an area code are split into 792 blocks of 10,000 numbers each. Telephone numbers are given to telephone companies in these large blocks. The biggest problem with this approach is that a lot of the numbers in a particular block may not be used by the company that owns them, but they are unavailable for use by anyone else. This means that the numbers in an area code with several different telephone companies and a growing population can be used up rather quickly.
When a new area code is needed, a decision must be made as to whether the existing area code should be split or the new area code overlaid on the existing one. A split means that the geographic region covered by the existing area code is reduced, and the new area code takes over the remainder. A split requires that a larger percentage of people change their phone number to reflect the new area code. An overlay is an area code that covers the same geographic region as the existing area code. The new overlay area code is given to new customers, and existing customers get to keep their current telephone number. The downside to overlay is that everyone in the geographic region must dial the entire 10-digit phone number, not just the prefix and line number.
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