Editing Overlay complex (section)

===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 this scheme was that if a number was in use in any portion of the Washington area, the corresponding number in Maryland or Virginia could only be used in an area considered a safe distance from the metro area, such as western Virginia or the [[Eastern Shore of Maryland]]. This system was ended in 1991 due to 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, Quebec|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, Quebec|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. As was the case in Washington, the only way to free up numbers was to end seven-digit dialing in the area, which was done in 2006. 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.