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 one or more existing area codes, resulting in two (or more) area codes serving the same area.
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Prior to the introduction of overlay plans, the method of introducing new area codes (also known as numbering plan areas or NPAs) in a region was to divide the existing territory of one area code into two (or more) pieces, allowing the more established or developed section to retain the original area code, and changing the numbering space of the other section(s) to have a new area code. For example, the original area code for the entire state of Washington was 206; today 206 applies to only the city of Seattle and immediate vicinity. This practice became known as a split plan.
In a number of cases, overlay plans were implemented on a special case basis to implement specialized dialing plans. In some areas, a party in one area code could dial a prefix which was local, but in a different area code, with only 7 digits. If they were calling a distant prefix in the same area code, they would either have to dial 1+ the number or 1+area code+ the number.
This practice was done on a large scale in the City of Washington, DC and its suburbs in Maryland and Virginia. Until 1991, any telephone in the Washington, DC metropolitan area could dial any local number in the District, Maryland or Virginia by dialing the 7-digit number. The way this was implemented was that every telephone in Maryland and Virginia that was in the Washington Metropolitan area was given a 'hidden' phone number consisting of the same number in the 202 area code, essentially making the 202 area code an overlay over the entire region. This meant a number in Virginia, actually in the 703 area code such as 931-xxxx could have been dialed as 703-931-xxxx or as 202-931-xxxx, while a Maryland number such as 585-xxxx which is actually in the 301 area code could be dialed as 301-585-xxxx or 202-585-xxxx. This also meant no prefix could be duplicated in any of the three jurisdictions. By 1991 the demand for more numbers was too great, the use of area code 202 as an overlay was discontinued, and callers in the area dialing an out-of-area-code number had to dial the full 10 digit number.
Urban sprawl accelerated the rate of expansion of metropolitan areas, and multiple split plans have caused the geographical area of a given area code in those regions to shrink. Also, the rapid growth in popularity of mobile phones, in addition to regular land line growth, has increased demand for new phone numbers even more.
The rise in popularity of mobile phones has added to the pressure against split plans, as an area code change affecting the exchange in which a cell phone is based requires the wireless carrier to reassign the number of every device based in those areas.
To alleviate complaints about such changes, in 1992 the telecommunication industry began to introduce "overlay plans" as a means to introduce new area codes. In this model, one fixed geographic area would concurrently have multiple valid area codes throughout. This plan's main benefit, which addressed many of the issues causing resistance to split plans, was that all existing phone numbers remained unaffected by the new area code. Newly-assigned numbers in the overlay plan areas would have the new area code. As a result, two telephones, located next to each other in the same building or house, could have different area codes. New York was the first area to employ this solution, using the 917 area code in addition to the older 212.
However, overlay plans introduced a new inconvenience: mandatory 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. 10-digit dialing is not a technical requirement but a U.S. Federal Communications Commission (FCC) mandate to promote fairness among telephone companies.[1]
Overlay plans have been used much more widely in some areas than others. For example, the northern third of Ohio is covered by two large overlay complexes, as is northern Georgia. Connecticut, Illinois, Oregon, New Jersey, Massachusetts, Maryland, and Texas have also used overlays heavily, and are (or soon will be) completely or mostly overlaid. In California, on the other hand, only four of 27 areas have overlays, and all were very recently implemented.
The first example of an entire state previously only served by a single code being overlaid was in West Virginia.
Overlays have also become more popular than split plans in Canada during the previous decade. All of British Columbia and Alberta, and large parts of Ontario and Quebec are currently overlaid. By 2013, all of Quebec, all of Manitoba and almost all of Ontario will be overlaid as well.
The North American Numbering Plan Administration recognizes different forms of overlays:
The persistent unpopularity of new area code creation, whether by split or overlay plans, led to a change in the rules of number block allocation, in order to conserve the pool of available phone numbers. This change, which allowed for the assignment of smaller number blocks, is commonly known as number pooling. This has noticeably slowed the need for area code growth, but not completely. For example, the Western Washington area narrowly avoided needing an overlay in 2001. Area code 564, originally planned for introduction in October 2001, was canceled in August 2001 after state regulators determined that the existing number pool had begun to be used more efficiently.