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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, replacing the existing area code within that section.
Split plans are used as growth in demand for telephone numbers (due to the increase in use of pagers, fax machines, and cellular phones) require new area codes in increasingly densely populated areas. After a split plan is implemented, the telephone numbers in an affected area are changed to a new area code, requiring printing of new stationery, advertisement, and signage, and the dissemination of new numbers to family, friends, and customers and their subsequent need to update speed dial programming and address books. Auxiliary devices such as cell phones, 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 this plan when adding area codes.