Overlay plan

Overlay plan

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.



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.

Rapid growth

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.

Reluctant solution

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.

Types of overlays

The North American Numbering Plan Administration recognizes different forms of overlays:

  • Distributed overlay (or all-services overlay): an entire existing area gains another area code serving the entire area. Most overlays are of this kind.
  • Single concentrated overlay: only the high-growth portion of an existing area gains a second area code.
  • Multiple concentrated overlay: the entire existing area gains multiple additional area codes, each of which serves a different subsection of the original. There are no known examples of such being implemented in the NANPA.
  • Multiple-area distributed overlay: two or more area codes gain a single new area code covering such an area. Examples include 872 in Chicago, Illinois (over 312 and 773) and 587 in Alberta (over 403 and 780).
  • Boundary-extension overlay: a neighboring area code (either an overlay code or single primary area code) is expanded to serve the area as well. Examples include 321 over 407 in central Florida and 778 over 250 in British Columbia.

Number pool management

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.

See also


External links

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Overlay — may refer to: Overlay architecture, term used to describe ‘event architecture’ and relates to the temporary elements that supplement existing buildings and infrastructure to enable the operation of major sporting events or festivals. Overlay… …   Wikipedia

  • North American numbering plan expansion — The North American numbering plan expansion is a proposed expansion of the North American Numbering Plan to accommodate future needs beyond the current 10 digit limitations used in telephone numbers.HistoryThe North American Numbering Plan has… …   Wikipedia

  • North American Numbering Plan expansion — The North American numbering plan expansion is a proposed expansion of the North American Numbering Plan to accommodate future needs beyond the current 10 digit limitations used in telephone numbers. Contents 1 History 2 Current industry… …   Wikipedia

  • Split plan — 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… …   Wikipedia

  • List of North American Numbering Plan area codes — This is a list of North American telephone area codes in effect for the North American Numbering Plan (NANP). The area to which an area code is officially assigned is known as a Numbering Plan Area (NPA). Contents 1 200 2 300 3 400 4 …   Wikipedia

  • North American Numbering Plan — NANPA redirects here. For other uses, see Nanpa (disambiguation). This article is about the numbering plan. For a list of area codes under the plan, see List of North American Numbering Plan area codes. The North American Numbering Plan (NANP) is …   Wikipedia

  • Heritage Overlay — A Heritage Overlay or HO is one of a number of planning scheme overlays contained in the Victorian Planning Provisions, for use in planning schemes in Victoria, Australia. The heritage overlay schedule of each local government planning scheme… …   Wikipedia

  • Maritime Industrial Zoning Overlay District — The Maritime Industrial Zoning Overlay District (MIZOD) was created in Baltimore, Maryland in 2004 to preserve deepwater access for port and maritime industrial uses. As waterfront residential and commercial development encroached on maritime… …   Wikipedia

  • Telephone numbering plan — Area Code redirects here. For the song by Ludacris, see Area Codes (song). A telephone numbering plan is a type of numbering scheme used in telecommunications to allocate telephone numbers to subscribers and to route telephone calls in a… …   Wikipedia

  • List of area code overlays — NOTOC An area code overlay is a North American area code which is located in the same geographic location as at least one other area code. They are determined and regulated as part of the North American Numbering Plan to increase the number of… …   Wikipedia