In telecommunications, an overlay plan is a telephone numbering plan that assigns multiple area codes to a geographic numbering plan area (NPA). Overlaying area codes is practiced in territories that belong to the North American Numbering Plan (NANP).
Since the creation of the North American Numbering Plan in 1947, new area codes are introduced by dividing an existing numbering plan area into multiple regions. One of those regions, usually the historically more established or developed place, retains the existing area code and requires no numbering changes in that area. That makes the central office codes of the other parts of the old numbering plan area available in that area, thus enlarging the number pool. (On its website, the North American Numbering Plan Administrator (NANPA) states, "Central office codes, also known as exchanges, prefixes, or simply NXXs, are digits 4, 5 and 6 of a ten-digit geographic NANP telephone number.") However, all subscribers in the newly-assigned area are required to update telephone number references, such as on letterheads and business cards and in directories. 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. The practice became known as a split plan.
In an overlay numbering plan, the change of the area code for numbers in parts of the existing numbering plan area is avoided by assigning additional area codes to the entire region of an existing code. 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. The overlay method generally required ten-digit dialing for all subscribers even to dial a local call that previously required only seven digits.
In several cases, however, 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 an office prefix that was local but in a different area code with only seven digits. If a subscriber called a distant office prefix in the same area code, the call would require dialing 1 and the seven-digit number or 1 and the ten-digit number, including the area code.
A similar practice was implemented on a large scale in the metropolitan area of Washington, DC, and its suburbs in Maryland and Virginia. Until 1991, the entire region was a single local calling area, and it was possible for anyone living in the metro area to dial a number in DC or the Maryland and Virginia suburbs with only seven digits. The entire area implemented a system of central office code protection in which no central office code was duplicated in the entire area. Each existing central office code was properly routed with each area code in the region so that each telephone number in the region could be dialed with any of the regional area codes, giving the appearance of an overlay plan.
A similar scheme was employed in Canada's capital, Ottawa. 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. Until 2006, it was possible to place a call between Ottawa and Hull with only seven digits. That continued even after Hull was merged into the larger city of Gatineau in 2002. That was implemented in a way that the same number could not be duplicated anywhere in eastern Ontario or western Quebec, even in areas a safe distance from the Ottawa area. However, Canada's inefficient number allocation system and the proliferation of cellphones brought 819 to the brink of exhaustion by the turn of the century. The only available numbers in 819 could have theoretically been used in the former Hull, but they could not be issued without ending seven-digit dialing between Ottawa and Hull. The scheme was largely ended in 2006, and the sole legacy of the old system is a "dual dialability" system for federal government numbers on both sides of the provincial border; all federal government offices on the Quebec side duplicated several exchanges worth of their counterparts on the Ontario side.
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, except in countries that assign shorter area codes and longer local numbers in areas with high population densities. The rapid growth in popularity of electronic devices (pagers and then mobile phones), in addition to regular landline growth, increased demand for new phone numbers even more. Although landline growth has sharply dropped and even decreased, largely by the elimination of residential landlines in favor of personal mobile phones, it has been replaced by the even worse problem of data-only devices (hotspots, modems, netbooks, and especially popular tablets), which still require a telephone number for use on cellular networks even if they are unable to make or receive regular calls.
The rise in popularity of mobile devices has added to the pressure against split plans, as an area-code change affecting the exchange in which a cellphone is based not only forces customers to reprogram their phones but also requires the wireless carrier to reassign the number of every device based in those areas. While phones with a SIM card have their own mobile device number updated automatically, there is still the inconvenience of users having to update their contact lists for local family members or others who have had their numbers changed too.
Most overlay plans introduced the inconvenience of mandatory ten-digit dialing in which the area code must always be included even for local calls. Ten-digit dialing is not a technically-necessary requirement, and 917 was initially deployed without it, but a US Federal Communications Commission (FCC) mandate instituted it at the demand of major telephone companies, to whom an overlay is considered a disadvantage to competitors. In Canada, ten-digit dialing is also a requirement of the Canadian Radio-television and Telecommunications Commission in overlay area codes.
Overlays initially were met with resistance, as they have resulted in different area codes existing within the same geographic area. In many cases, such as 847 in northwestern Chicago and 212 in New York City, an overlay was an additional disruption to a community that had already been subject to one or multiple code splits, encountering pushback from state regulators or consumer groups. However, overlay plans were eventually used much more widely in some areas than others. For example, much of Ohio is covered by large overlay complexes, as is northern Georgia. Connecticut, Illinois, Oregon, New Jersey, Massachusetts, Maryland, Pennsylvania, Texas, and, more recently, California have also implemented many overlays. 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. Seven-digit dialing has ended in most area codes serving major cities. Among the few cities still allowing seven-digit dialing are Detroit, El Paso, Jacksonville, Louisville, Memphis, Milwaukee, and Oklahoma City. Even in those cities, seven-digit dialing may be possible only within the area code that serves the city itself, not in outlying areas with different area codes.
Telecommunications companies have increasingly favored overlays even in sparsely-populated rural areas in which ten-digit local numbers are unnecessary, as split plans force cellular providers to reprogram millions of client handsets to reflect changes in existing mobile numbers. Customers also incur costs to publish new letterheads and to reprogram stored address book data on individual devices. They have become even more popular as the proliferation of cell phones has caused area codes to exhaust fairly quickly. That is especially the case in area codes that have been pushed back to the brink of exhaustion after being recently split, as carriers want to keep their customers from having to change their numbers for the second time in a decade or less.
Overlay plans also favor incumbent wireline carriers over new entrants, as the established firm already has large allocations of numbers in the more desirable existing code, and subscribers of new or growing competing carriers are relegated to unfamiliar, new codes.
The first example of an entire state served by only a single until it was overlaid is West Virginia, which had been served with area code 304 since the inception of the North American Numbering Plan. At first, state officials voted to split off northern West Virginia with area code 681 and to leave southern West Virginia in 304. However, lobbying by the telecommunications industry led the state to reverse the decision and turn 681 into an overlay. Idaho followed that precedent in 2017 by overlaying area code 208 with 986.
Overlays gained popularity among Canadian telephone companies in the early 2000s, primarily as a workaround for the country's inefficient number allocation system. Every competing carrier is issued blocks of 10,000 numbers, the maximum for each corresponding three-digit prefix, in each rate center in which it offers new service and each local interconnection region in which it ports existing numbers. Most rate centres in Canada do not need nearly that many numbers to service customers adequately, but a number cannot be reassigned elsewhere once it is assigned to a carrier and rate centre. That resulted in a large amount of unallocated numbers, which was exacerbated by the proliferation of cellphones, fax machines, pagers, and dial-up Internet connections, particularly in larger markets. 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 number allocation problem manifested itself at the turn of the century in Canada's largest rate center, Metro Toronto. The area code for Metro Toronto, 416, was on the verge of exhaustion only five years after the suburbs had split off as area code 905 in 1993. American cities such as New York City, Chicago, and Los Angeles had been split between two area codes, but that solution was quickly ruled out for Toronto because of the area's extremely dense population and the lack of a suitable boundary for a split. It was ultimately decided to overlay 416 with area code 647 in 2001, two years after Metro Toronto was merged into the "megacity" of Toronto. The successful implementation of 647 triggered the rapid adoption of overlays across Canada. By 2013, seven-digit dialing had been eliminated in every Canadian numbering plan area except in the country's three Arctic territories (area code 867), a large but sparsely-populated area in northwestern Ontario (area code 807), New Brunswick (area code 506), and Newfoundland and Labrador (area code 709). Overlays are now the preferred method of expansion in Canada, and no numbering plan areas have been split in the country since 1999.
Recently, British Columbia and the Prairie Provinces have followed West Virginia's lead and implemented province-wide overlays. In those cases, a factor behind the decision was the desire to spare rural areas the expense and the burden of changing their numbers.
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. This example was caused by area code 564 serving Washington except 509 in 2017.
- Multiple concentrated overlay: the entire existing area gains multiple additional area codes, each of which serves a different subsection of the original. This has never occurred yet.
- 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) as well as 587 in Alberta (over 403 and 780).
- Boundary-extension overlay: a neighboring area code (an overlay code or a 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. In 1999, 281 was the first to do so, merging with 713 in Houston; later that year, 972 merged with 214, followed by 323 merging with 213 in Los Angeles in 2017 and 858 with 619 a year later.
- Service-specific overlay: the first overlay area code in the NANPA, 917, is the only example of this; it was established originally as an area code specifically for cellphones and pagers in New York City, but soon afterward, the FCC said that area codes going forth could not be service-specific, but it allowed 917 to remain as such. However, 917 is now used for landlines in New York City on a limited basis.
Number pool management
The persistent unpopularity of new area code creation by split or overlay plans has led to a change in the rules of number block allocation to conserve the pool of available telephone numbers. 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. For example, the western half of Washington, including Seattle, narrowly avoided the need for 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 use of number pooling had allowed existing numbers in western Washington to be used more efficiently. However, the overlay finally occurred in 2017.
Number pooling is not practiced in Canada. As was previously mentioned, every competing carrier is issued blocks of 10,000 numbers for every rate center in which it offers service, regardless of its actual subscriber count. Individual rate centers exist even in the smallest hamlets, and even tiny unincorporated villages receive multiple blocks of 10,000 numbers. Larger cities, particularly "megacities" that were created by amalgamations in the 1990s and early 2000s, have multiple rate centers, which were not combined for years, if at all. For example, Mississauga, Canada's sixth-largest city, is split between five rate centers even though it has been a single municipality since 1974. Hamilton, the nation's tenth-largest city, is split between nine rate centers. Surrey, the nation's twelfth-largest city, is split between four rate centers, while Ottawa is split between 11. Overlays became the preferred relief solution in Canada, partly because it is not possible to reassign a number from a smaller rate center to a larger one, even if the smaller rate center has enough numbers to serve it. By 2013, as mentioned above, seven-digit dialling had been eliminated in all eight of Canada's original numbering plan areas.
- "NANPA : Numbering Resources - Central Office Codes". nationalnanpa.com. Retrieved October 10, 2019.
- FCC. "Area Codes: Frequently Asked Questions". Archived from the original on June 15, 2006. Retrieved June 22, 2006.