Central office code protection

exchange code protectioncode protection
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.wikipedia
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Seven-digit dialing

7-digit dialing7-digit diallingfixed-length seven-digit number
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 made it possible in some low density areas to use seven digits to reach areas in another area code.

Ten-digit dialing

10-digit dialingten-digit diallingten digits
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.
Some communities on an area code boundary, such as Ottawa-Hull (613/819), Kansas City MO/KS (816/913) or Washington, D.C. (202) implemented central office code protection to ensure the same seven-digit local number was not assigned in two different area codes in the same city to retain seven-digit dialing in the entire community.

North American Numbering Plan

Area codeArea codesArea code(s)
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.
Allowing 7D local dial across an area code boundary, which is uncommon today, requires central office code protection, locally if using toll alerting, across the entire area code otherwise, to avoid assignment of the same seven-digit number on both sides.

Overlay plan

overlayoverlaidoverlay area code
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 entire area implemented a system of central office code protection in which no central office code was duplicated in the entire area.

Area codes 613 and 343

613613, 343343
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.
To preserve seven-digit dialling between Ottawa and Hull, an exchange code protection scheme was implemented so that the same seven-digit local number could not be assigned on both sides of the National Capital Region.

Area codes 819 and 873

819area code 819819, 873
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.
This code protection was implemented in such a manner that, if an exchange in +1-819-77x (1-819-PRovince) was in use in Hull, the corresponding +1-613-77x exchange could not be used anywhere in Eastern Ontario.

Area codes 301 and 240

301240 & 301240
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.
From 1947 to 1990, the telephone numbering plan of the entire Washington metropolitan area, including Washington, D.C. (area code 202), and the large suburban areas in Maryland and Virginia (area code 703), operated in a system of central office code protection, meaning that no central office code in Washington D.C., and in the Maryland and Virginia suburbs could exist in more than a single central office.

Linked numbering scheme

linked-number area
In some cases, a local call to a number just across an area code boundary could be seven digits if an exchange code protection scheme prevented the same prefixes being assigned to local numbers in the other area code.

Telephone numbering plan

Area codeTelephone codearea codes
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.

Competitive local exchange carrier

CLECcompeting carrierCompetitive Local Exchange Carriers
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.

Bell Aliant

AliantBell Aliant TVAliant Telecom
As an example, the entire 506-752 prefix is assigned to Aliant Telecom (NB) and serves Campobello Island, New Brunswick.

Campobello Island

CampobelloCampobello Island, New BrunswickCampobello Island, NB
As an example, the entire 506-752 prefix is assigned to Aliant Telecom (NB) and serves Campobello Island, New Brunswick.

New Brunswick

NBProvince of New BrunswickNew Brunswick, Canada
As an example, the entire 506-752 prefix is assigned to Aliant Telecom (NB) and serves Campobello Island, New Brunswick.

Lubec, Maine

Lubec
Campobello is a border community and is a local call to Lubec, Maine, in area code 207.

Area code 207

207telephone area codeArea codes in Maine
Campobello is a border community and is a local call to Lubec, Maine, in area code 207.

FairPoint Communications

FairPointFairPoint Communications, Inc.
The 207-733 prefix serves Lubec, split between FairPoint Communications and a pair of competitors using number pooling.

Number pooling

The 207-733 prefix serves Lubec, split between FairPoint Communications and a pair of competitors using number pooling.

Area code 506

506
Both area code 506 and 207 use seven digits for local calls.

Massachusetts

MACommonwealth of MassachusettsMass.