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In telephony, especially in North America, an overlay plan is the practice of introducing a new area code by applying it onto a geographic area that is already occupied by an existing area code.
Prior to the introduction of overlay plans, the method of introducing new NPAs (area codes) in an area was to divide the existing area of one NPA into two (or more) pieces, allowing the more established or developed section to retain the original NPA, and changing the numbering space of the other section(s) to have a new NPA. For example, the original NPA for the entire state of Washington was 206; today 206 only applies to the city of Seattle and immediate vicinity.
As urban sprawl accelerated the rate of expansion of metropolitan areas, and as NPA areas themselves became smaller in those areas, the difficulty of changing area codes (mostly adminsitrative, such as changes in stationery, advertising, and communicating the change to friends and customers), even in the less established part of an area code, made this practice unpopular.
To alleviate complaints about such changes, in the late 1990s, the telecommunication industry began to introduce "overlay plans" as a means to introduce new NPAs. In this model, one fixed geographic area would concurrently have multiple valid NPAs throughout. This plan's main benefit, which addressed much of the issues causing resistance to split plans, was that all existing phone numbers remained unaffected by the new NPA. Newly-assigned numbers in the overlay plan areas would have the new area code. As a result, two telephones, located next to each other, could have different area codes.
However, overlay plans introduced a new inconvenience: the need for constant 10-digit dialing (i.e. the area code must be included), even for local calls, in the affected area. This and the difficulty of remembering all the area codes in a geographic area made overlay plans only marginally less unpopular than split plans.
This persistent unpopularity over new NPA creation encouraged the industry to implement new number block allocation rules in order to conserve the pool of available phone numbers. This has noticeably slowed the need for NPA growth, but not completely. For example, the Western Washington area narrowly avoided needing an overlay NPA in 2001. NPA 564, originally planned for introduction in October 2001, was cancelled in August 2001 after state regulators determined that the existing number pool had begun to be used more efficiently.
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