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. Overlays have especially been favored for numbering plan areas that had grown substantially to the point of needed further relief, after being split, as carriers want to minimize additional customer inconveniences. For example,
Maryland had been served solely by
area code 301 from the implementation of the area code system until 1990, when the area from
Baltimore eastward was split off with
area code 410. Within only five years, 410 was already threatend by central office code exhaustion from the proliferation of cell phones, pagers, fax machines, and dial-up Internet lines. The area's dominant telephone provider, Bell Atlantic (now part of
Verizon) realized that a split would have forced residents of either Baltimore or the Eastern Shore to change their numbers for the second time in the 1990s. Accordingly, area code 443 was created as an overlay for all of the eastern portion of Maryland in 1996. 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 area code overlaid on an entire U.S. jurisdiction was
Puerto Rico, with area code 939 added to the numbering plan area of area code 787, in 2001. Puerto Rico had just been split from
area code 809, the area code for much of the Caribbean, in 1996. A split was deemed unfeasible because of the island's extremely dense population and the lack of a suitable boundary for a split. Additionally, a split would have forced half of Puerto Ricans to change telephone numbers for the second time in a decade. The first instance of a
statewide overlay was
West Virginia's
area code 304, in 2009. West Virginia had been served by 304 since the inception of the
North American Numbering Plan, but by 2007 the state needed a new area code. At first, state officials voted to split off northern West Virginia with area code 681 and to leave southern West Virginia with 304. However, lobbying by the telecommunications industry led to a state-wide overlay.
Idaho followed that precedent in 2017 by overlaying
area codes 208 and 986. Overlays gained popularity in the Canadian telephone network in the early 2000s as a more efficient relief method. The proliferation of cellphones, fax machines, pagers, and dial-up Internet connections, particularly in larger markets, threatened central office code exhaustion in many areas. The area codes for four of the five largest Canadian markets (
416 in
Toronto,
514 in
Montreal,
604 in
Vancouver, and
403 in
Calgary) had previously been split in the 1990s. Particularly severe allocation problems manifested themselves at the turn of the century in Canada's largest local calling area,
Metro Toronto and the
Golden Horseshoe. This area had been served by 416 since the institution of the area code system. The Golden Horseshoe's explosive growth in the second half of the 20th century makes numbers tend to be used up fairly quickly. As a result, the number allocation problem was not as severe in 416 as it was in the rest of Canada. Nevertheless, it was severe enough to force Toronto's suburban ring into
area code 905 in 1993. While the Golden Horseshoe would have likely needed another area code because of its continued growth, it is very likely that the immediate need for relief would have been staved off had it been possible to reallocate numbers from the smaller rate centers in the region to Metro Toronto. Within two years of the 905 split, 416 was back on the verge of exhaustion. 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 Metro 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 2023, seven-digit dialing had been nearly eliminated nationwide in Canada, and no numbering plan areas have been split in the country since 1999. Overlays have become the preferred method of expansion in Canada, They spare rural areas the expense and burden of changing telephone numbers. As a result,
British Columbia and the
Prairie Provinces have installed province-wide overlays. ==Number pool management==