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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.〔AT&T (1968), ''Notes on Distance Dialing'', p.10〕〔AT&T (1980), ''Notes on the Network'', p.2-4〕 Central office code protection was once common in communities such as Ottawa-Hull〔http://www.crtc.gc.ca/eng/archive/2001/PT2001-71a.htm〕 or Washington, D.C. which were constructed on provincial or state boundary lines. It is 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. ==Operation== The North American Numbering Plan allocates telephone numbers to central offices in blocks of 10000 consecutive numbers; each block is identified by area code (three digits) and central office code (three digits). The remaining four digits identify the line number within the exchange. These blocks are subdivided and assigned in groups of 1000 to multiple local service providers. As an example, the entire 506-752 prefix is assigned to Aliant Telecom (NB) and serves Campobello Island, New Brunswick.〔http://cnac.ca/data/COCodeStatus_NPA506.txt〕 Campobello is a border community〔(【引用サイトリンク】title=Campobello's sea change )〕 and is a local call to Lubec, Maine〔(【引用サイトリンク】title=Local calling guide: Rate centre information, Campobello )〕 in area code 207. The 207-733 prefix serves Lubec, split between FairPoint Communications and a pair of competitors using number pooling.〔(【引用サイトリンク】title=Local calling guide: NPA-NXX search, 207-733 )〕 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.〔(【引用サイトリンク】title=Local calling guide: NPA-NXX search, 506-733 )〕 Similarly, Edmundston is local to Madawaska, Maine exchanges 207-316, 207-436 and 207-728.〔(【引用サイトリンク】title=Local calling guide: Rate centre information, Madawaska )〕 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〔http://cnac.ca/data/COCodeStatus_NPA506.txt〕 A few small towns on US state boundaries have one central office which serves both sides, using different prefixes in different area codes.〔http://massis.lcs.mit.edu/telecom-archives/TELECOM_Digest_Online2006-1/1065.html〕 All South Coffeyville, Oklahoma exchange prefixes are served from Coffeyville, Kansas, population 10000;〔http://www.localcallingguide.com/lca_prefix.php?switch=CFVLKS10DS0〕〔http://www.localcallingguide.com/lca_prefix.php?switch=PRSSKSBL6MD〕 the two are local to each other but long distance to everything else. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Central office code protection」の詳細全文を読む スポンサード リンク
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