Overlay plan

overlayoverlaidoverlay area codeoverlay codeconcentrated overlayarea code overlayoverlayssharing the service areacode overlaycover the same geographic region
In telecommunications, an overlay plan is a telephone numbering plan that assigns multiple area codes to a geographic numbering plan area (NPA).wikipedia
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Area codes 214, 469, and 972

972214469
Area code 469 was introduced on July 1, 1999 in an overlay plan for most of the eastern portion of the Metroplex.

Area codes 619 and 858

619858area code 619
After the 619 area code reached a projected exhaust date of 2019, the California Public Utilities Commission approved a boundary merging overlay plan that merged area codes 619 and 858.

Area code 575

575
There have been no area code splits since 2007, when area code 575 split off 505 in New Mexico, and no splits are currently proposed, but many overlays are.
All new area codes added in the U.S. since the introduction of 575 have been overlays, in which the new code serves the same geographic area as the previous code.

List of Oregon area codes

listOregon
Connecticut, Illinois, Oregon, New Jersey, Massachusetts, Maryland, Pennsylvania, Texas, and, more recently, California have also implemented many overlays.
971: An overlay of area code 503.

Area codes 213 and 323

323213area code 213
Since 213 still had an abundance of numbers available, the California Public Utilities Commission approved a plan that erased the boundaries between both area codes, converting the area to an overlay plan for all of central Los Angeles.

Number pooling

The change, which allowed for the assignment of smaller number blocks, is commonly known as number pooling and has noticeably slowed the need for area code growth.
The first deployment of an overlay plan with forced ten-digit dialing for local calls on June 1, 1997, in Maryland's area code 301, also led to consumer confusion as the same point could now have multiple area codes.

North American Numbering Plan

Area codeArea codesArea code(s)
Overlaying area codes is practiced in territories that belong to the North American Numbering Plan (NANP).
Area codes are added by two principal methods, number plan area splits and overlay plans.

Area code 403

+1-403403-1+403
Those are some of the reasons that the numbering plan areas corresponding to four of the five largest Canadian markets (416 in Toronto, 514 in Montreal, 604 in Vancouver, and 403 in Calgary) were split in the 1990s.
The 403 area code, simultaneously with area code 780, will be overlaid with the new area code, which will cover the entire province.

Area code 780

+1-780780-1+780
The 780 area code, simultaneously with area code 403, was overlaid with the new area code, which will cover the entire province.

Telecommunication

telecommunicationscommunicationstelecom
In telecommunications, an overlay plan is a telephone numbering plan that assigns multiple area codes to a geographic numbering plan area (NPA).

Washington (state)

WashingtonWashington StateWA
For example, the original area code for the entire state of Washington was 206, which, however, now applies only to the city of Seattle and the immediate vicinity.

Seattle

Seattle, WashingtonSeattle, WACity of Seattle
For example, the original area code for the entire state of Washington was 206, which, however, now applies only to the city of Seattle and the immediate vicinity.

New York City

New YorkNew York, New YorkNew York City, New York
The first use of that solution was in New York City, where area code 917 was overlaid onto Manhattan's 212 and the outer boroughs' 718.

Manhattan

Manhattan, New YorkManhattan, New York CityNew York
The first use of that solution was in New York City, where area code 917 was overlaid onto Manhattan's 212 and the outer boroughs' 718.

Area codes 718, 347, and 929

718, 347, 929718347
The first use of that solution was in New York City, where area code 917 was overlaid onto Manhattan's 212 and the outer boroughs' 718.

Washington, D.C.

Washington, DCWashington D.C.District of Columbia
A similar practice was implemented on a large scale in the metropolitan area of Washington, DC, and its suburbs in Maryland and Virginia.

Maryland

MDState of MarylandMaryland, USA
A similar practice was implemented on a large scale in the metropolitan area of Washington, DC, and its suburbs in Maryland and Virginia.

Virginia

Commonwealth of VirginiaVAState of Virginia
A similar practice was implemented on a large scale in the metropolitan area of Washington, DC, and its suburbs in Maryland and Virginia.

Ottawa

Ottawa, OntarioOttawa, CanadaOttawa, ON
A similar scheme was employed in Canada's capital, Ottawa.

Hull, Quebec

HullHull sectorHull, Québec
It shared the same calling area as Hull, its twin city in Quebec, even though Ottawa was in area code 613 and Hull was in area code 819.

Quebec

QuébecProvince of QuebecQC
It shared the same calling area as Hull, its twin city in Quebec, even though Ottawa was in area code 613 and Hull was in area code 819.

Area codes 819 and 873

819area code 819819, 873
It shared the same calling area as Hull, its twin city in Quebec, even though Ottawa was in area code 613 and Hull was in area code 819.

Gatineau

Gatineau, QuebecGatineau, QCCity of Gatineau
That continued even after Hull was merged into the larger city of Gatineau in 2002.