In the administration of the North American Numbering Plan, central office code protection was a numbering policy intended to ensure that the same local telephone number was not assigned in both of a pair of adjacent communities on opposite sides of an area code boundary. Its primary purpose was initially to preserve seven-digit dialing of local calls across a common area code boundary in border communities.
Central office code protection was once common in communities on provincial or state boundary lines. It has been declining in use as inefficient allocation of numbering resources to the growing number of competitive local exchange carriers has caused depletion of available number prefixes, often requiring ten-digit local calls or overlay plans where multiple area codes serve the same geographic location.
The North American Numbering Plan allocates telephone numbers to central offices in blocks of 10000 consecutive numbers; each block is identified by the area code (three digits) and a central office code (three digits). The remaining four digits identify the line number within the exchange. For allocation to local service providers, these blocks are subdivided and assigned in groups of 1000.
As an example, the entire 506-752 prefix is assigned to Aliant Telecom (NB) and serves Campobello Island, New Brunswick. Campobello is a border community and is a local call to Lubec, Maine in area code 207.
The 207-733 prefix serves Lubec, split between FairPoint Communications and a pair of competitors using number pooling. Both area code 506 and 207 use seven digits for local calls.
A seven-digit local call from Campobello to Lubec is possible, provided that 733-XXXX is not assigned to anything in the 506 area code which is local anywhere near Campobello. This may be done in one of two ways:The Canadian numbering administrator could mark +1-506-733 as "reserved" and refuse to issue it at all. This was originally the standard procedure.
The 506-733 prefix could be dumped as far out-of-region as possible within the 506 area code boundary, if calls to the other side of the same code are long-distance and a trunk call within the area code requires the entire number (including the leading 1-506) be dialled. By dumping the 506-733 prefix in Edmundston, the geographically-furthest point still in New Brunswick, the administrator adopts this approach.
Similarly, Edmundston is local to Madawaska, Maine exchanges 207-316, 207-436 and 207-728. The corresponding 506-316, 506-436 and 506-728 prefixes (as of 2014) are not issued by the Canadian Numbering Administrator (CNAC):
506,316,,,Available,,Available outside Madawaska: Maine EAS
506,436,,,Available,,Available outside Madawaska: Maine EAS
506,728,,,Available,,Available outside Madawaska: Maine EAS
A few towns and suburbs on US state boundaries have one central office which serves both sides, using different prefixes in different area codes.All South Coffeyville, Oklahoma exchange prefixes are served from Coffeyville, Kansas, population 10000; the two are local to each other but long distance to everything else.
The towns of Seekonk and Rehoboth, Massachusetts share a wire center with East Providence, Rhode Island and code protection was provided in the 401 area code until 2000, allowing Seekonk and Rehoboth subscribers (336-xxxx) to dial calls in the Providence, Rhode Island metro area with 7 digits. This was phased out when New England Telephone began the process of creating an overlay area code for the 508 area code. Ten-digit dialing is now required for calls crossing the area code (and state) boundary.
If code protection is implemented by reserving every seven-digit number for a border town in both (or all) of the affected area codes, that community and points in its local calling area occupy numbering resources at double the otherwise-expected rate. This is a minor drawback in small cities with large rural area codes (as the numbers can easily be assignable in distant locations), but can consume numbers rapidly in larger centres such as St. Louis or Kansas City (where such protection must be maintained throughout most or all of the area code region, reducing its lifespan).
In large cities located directly on area code boundaries (such as Ottawa-Hull in 613/819, the sixth-largest Canadian metropolitan area), this can lead to a situation where none of the many vacant prefixes can be assigned without breaking seven-digit calling across the area code boundary. Code protection in Ottawa-Hull broke down in 2006 for this reason. The 1-819 versions of Ottawa 1-613 numbers could not be assigned anywhere in western Quebec, even to areas a safe distance from the National Capital Region, such as the Eastern Townships or northwestern Quebec. Eventually, the few remaining prefixes in area code 819 could not be assigned without requiring ten-digit dialing from Ottawa to Hull.
A local calling area spanning three American or Canadian jurisdictions would require any number local to any part of the town to be reserved across all three (or more) area codes. This ultimately led to the breakdown of seven-digit calling between Washington, D.C. and its suburbs in 1991. Since the Washington local access and transport area (LATA) spills into portions of Maryland and Virginia, every number in the area that was in Maryland's area code 301 and northern Virginia's area code 703 was given a "hidden" number in the District's area code 202. However, this meant that if a central office code was in use in any portion of the metro area, it could not be used even in areas a safe distance from Washington such as southwestern Virginia or the Eastern Shore of Maryland.
Even if a city is not directly on an area code boundary, it may require code protection if a suburb in its local calling area can call another area code locally. Chicago is long distance to any point in Indiana, even though the Chicago LATA stretches for some distance into northwestern Indiana. However, Calumet City, Illinois is a local call to East Chicago, Indiana. A dial plan which uses purely seven-digit local calling from Calumet City would protect a massive number of Chicago 312 numbers from assignment in Indiana's 317, which forced northern Indiana to split off as area code 219 in the first year (1948) after the original area codes were assigned (1947).
Severing the border community from its exhausted home area code using a split plan may prolong the life of a seven-digit exchange code protection scheme if the piece being split off is outside the extended local calling area. Code protection is most often removed entirely in large cities if an overlay plan is implemented, as these plans break the seven-digit dialling that code protection was devised to preserve. The plan also can be expected to break down if new split-plan codes are added within a city the size of Chicago, as the city itself no longer fits into one seven-digit area.