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, replacing the existing area code within that section.
Split plans were used in the 1980s and 1990s, as growth in demand for telephone numbers (due to the increase in use of pagers, fax machines, and cellular phones) required new area codes in increasingly densely populated areas. Unpopularity resulted from the fact that, after a split plan was implemented, the telephone numbers in an affected area were 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.
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.