In telecommunications, an area code overlay complex is a telephone numbering plan that assigns multiple area codes to the same geographic numbering plan area (NPA). Area code overlays are implemented in territories of the North American Numbering Plan (NANP) to mitigate exhaustion of central office codes in growth areas. The method has been in use since 1992, and has been the exclusive method of area code relief since 2007.
History From the 1947 inception of the North American Numbering Plan in to 1992, the only method of introducing new area codes was the area code split. It divided an existing numbering plan area (NPA) into multiple regions, one of which retained the existing area code. The other parts were each assigned a new NPA code.
The retaining area was usually the historically more established or developed place, and required no numbering changes (one exception was area code 606 in eastern Kentucky; the populous areas of Lexington and the Cincinnati suburbs in northern Kentucky received the new area code 859, with other locations, including the Eastern Kentucky Coalfield, retaining 606). It gained numbering capacity by acquiring the central office codes of the other areas, because those were delegated to a new area code in the process. The existing central office codes in the separated areas are maintained in the new numbering plan area and so that the change for existing subscribers is the change of area code. Thus, only area code references, where present for long-distance calling, required updating in directories, letterheads, and business cards. Local seven-digit dialing was unaffected by the area code change.
With the proliferation of electronic switching systems starting in the 1980s, it became possible to implement another method for expanding numbering plan resources, the area code overlay. In an overlay numbering plan, also called overlay complex, an additional area code is assigned to an existing numbering plan area, thereby avoiding the need for existing customers in the reassigned regions to change telephone numbers. The first use of that solution was in New York City in 1992, when area code 917 was overlaid to two numbering plan areas: area code 212 in Manhattan and area code 718 in the outer boroughs.
Types The North American Numbering Plan Administrator recognizes several types of overlays:
In a distributed overlay, or all-services overlay, the most common type, an entire existing numbering plan area (NPA) gains another area code serving the entire area.
In a single concentrated overlay, only the high-growth portion of an existing area gains a second area code. This was previously implemented in several numbering plan areas, but all of them have since been expanded to the entire NPA.
In a multiple concentrated overlay, the entire existing numbering plan area gains multiple additional area codes, each of which serves a different subsection of the original. This has never been implemented.
In a multiple-area distributed overlay, two or more numbering plan areas gain a single new area code covering the entire area, either concurrently or in phases. Examples include 872 in Chicago, Illinois (over 312 and 773) as well as 587 and 825 in Alberta (over 403 and 780). In a phased example, area code 564 was implemented initially only on area code 360 in 2017, but will expand to three other codes when needed, in western Washington.
A boundary-extension overlay is a type in which one or more neighboring numbering plan areas is expanded. Examples include 321 over 407 in central Florida and 778 over 250 in British Columbia. In 1999, 281 was the first of this type, merging with 713 in Houston. Later that year, 972 merged with 214 in Dallas, followed by 323 merging with 213 in Los Angeles in 2017, and 858, in San Diego, with 619 a year later. In 2023, the first multiple boundary-extension overlay was implemented with the folding of area codes 602, 480, and 623 into a single overlay complex for the Phoenix metropolitan area.
Service-specific overlays are not practiced in the NANP. Only one case, the first overlay area code in the NANPA, 917, was ever an example. It was established originally as an area code specifically for cellphones and pagers in New York City, but soon afterward, the FCC specified that area codes must not be service-specific though it grandfathered the use for 917. Later, 917 became available for landline assignments in New York City.
Methodology Implementations of the overlay method requires ten-digit dialing, by including the area code, for all calling destinations, local or long-distance. The only notable, temporary, exception was the introduction of 917 overlaying all of New York City for mobile services, which initially kept seven-digit dial within 212.
Exceptions Near numbering plan area boundaries, special situations arose at times when established communities extended on both sides of the boundary. To preserve local community integrity, identity, and convenience, a party could dial an office prefix for a local call with only seven-digits, but the destination was technically located in the adjacent number plan area. If a subscriber called a distant office prefix with the same area code, the call would require the dialing prefix 1 before the seven- or ten-digit number.
A similar practice was implemented on a large scale in the metropolitan area of Washington, DC, and its suburbs in Maryland and Virginia. Most of the Washington region's inner ring is a single local calling area, even though it is split between three area codes–the District's area code 202, Maryland's area code 301 and Northern Virginia's area code 703. For most of the second half of the 20th century, it was possible to dial any number in the metro area with just 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. One side effect of the scheme was that if a number was in use in any portion of the Washington area, the corresponding number in Maryland or Virginia could be used only in an area that was considered a safe distance from the metro area, such as Western Virginia or the Eastern Shore of Maryland. This system was ended in 1991 because of an impending number shortage on both sides of the Potomac River.
A similar method 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 was implemented with central office code protection so that the same seven-digit telephone number could not be duplicated anywhere in Eastern Ontario or Western Quebec, even in areas a safe distance from the Ottawa area. The scheme briefly continued after Hull was merged into the larger city of Gatineau in 2002. However, by the turn of the century, 819 was threatened with exhaustion by the increased demand for numbers brought on by the proliferation of cell phones. By then, the only available prefixes in 819 could theoretically be used in the former Hull but only by breaking seven-digit dialing between Ottawa and Hull. That was the result in 2006, when ten-digit dialing became mandatory in both NPAs. The sole legacy of the old configuration is the duplication of federal government numbers on both sides of the provincial border with differing area codes.
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, 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.
Compromise 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.
Popularity Overlays were initially met with resistance, as they result in multiple area codes in the same geographic area, which requires ten-digit dialing. In many cases, such as 847 in Chicago's northwestern suburbs 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 has large overlay complexes, as has northern Georgia and the North Carolina Piedmont. Connecticut, Illinois, Oregon, New Jersey, Massachusetts, Maryland, Pennsylvania, Texas, Virginia and, more recently, California have also implemented many overlays. No area code splits have been performed since 2007, when area code 575 split off 505 in New Mexico, and no splits are proposed.
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. Customers also incur costs to publish new letterheads and to reprogram stored address book data on individual devices. Overlays have become more expedient as the proliferation of cell phones has accelerated area codes exhaustion. In most relief planning projects, overlays have become the sole option considered in the planning stage.