Area code split
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In telephony, a split plan is the practice of introducing a new area code by dividing an existing area code's territory and applying the new area code to one of the resulting divisions, thus replacing the existing area code within that section.
The increasing demand for telephone numbers is caused by the increase in use of pagers, fax machines, and cellphones, which requires new area codes in increasingly-densely-populated areas.
After a split plan 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 second half 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 introduced overlay plans, which apply multiple area codes to the same area. They allow customers to keep existing phone numbers but require 10-digit dialing in the affected area. Since 2007, many territories use overlays when they add new area codes.