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In telecommunications, a split plan is the practice of introducing a new telephone area code by dividing an existing numbering plan area (NPA), and assigning a new area code to one of the resulting divisions, and retaining the existing area code only for one area, which is typically the traditionally more developed or established place. The practice serves the purpose of preventing telephone number exhaustion in regions of high growth. The increasing demand for telephone numbers has been primarily caused by increase in use of pagers, fax machines during the 1990s, and proliferation of cellphones and smart phones since.
After a numbering plan split is implemented, the telephone numbers in the affected area are changed to a new area code, which requires the printing of new stationery, advertisements, and signage; the dissemination of the new numbers to family, friends, and customers; and the subsequent need to update speed dial programming and address books. Auxiliary devices, such as cellphones, pagers, and fax machines, must be reprogrammed when an area code splits. In a number of areas, rapid growth during the late decades of the 20th century resulted in several splits within a short time, in some cases within a decade.
As a result, in the mid-1990s, the North American Numbering Plan Administrator introduced another method called overlay plan, which assigns multiple area codes to the same numbering plan area. This permits customers to keep existing telephone numbers, but it requires ten-digit dialing for customers of both area codes. Since 2007, most territories use overlays for mitigating numbering shortages.