Central office code protection
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In the administration of the
North American Numbering Plan The North American Numbering Plan (NANP) is a telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. This group is historically known as World Zone 1 and has the international callin ...
, central office code protection is a numbering policy for maintaining local
seven-digit dialing Seven-digit dialing is a telephone dialing procedure customary in some territories of the North American Numbering Plan (NANP) for dialing telephone numbers in the same numbering plan area (NPA). NANP telephone numbers consist of ten digits, of wh ...
in communities that extend on both sides of the boundary line between multiple
numbering plan area The North American Numbering Plan (NANP) is a telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. This group is historically known as World Zone 1 and has the international calling ...
s (NPAs), such as in cross-border towns on state lines. Code protection prevents the use of the same telephone number on both sides by not assigning the same central office code of one NPA in the adjacent NPA.AT&T (1980), ''Notes on the Network'', p.2-4 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 carrier A competitive local exchange carrier (CLEC), in the United States and Canada, is a telecommunications provider company (sometimes called a " carrier") competing with other, already established carriers, generally the incumbent local exchange carrie ...
s has caused depletion of available number prefixes, often requiring ten-digit local calls and
overlay plan Overlay may refer to: Computers *Overlay network, a computer network which is built on top of another network *Hardware overlay, one type of video overlay that uses memory dedicated to the application *Another term for exec, replacing one process ...
s where multiple area codes serve the same geographic location. Despite its advantages where a community of interest reaches across an NPA boundary, central office code protection was only acceptable as long as it could be continued without the threat of central office code exhaustion in the NPA protecting the code.(COCAG)'' Despite its advantages where a community of interest reaches across an NPA boundary, central office code protection was only acceptable as long as it could be continued without the threat of central office code exhaustion in the NPA protecting the code.


Examples

The entire 506-752 prefix is assigned to
Aliant Telecom Bell Aliant is a brand name used by Bell Canada for Telecommunications in Canada, telecommunications services in Atlantic Canada. Prior to 2015, Bell Aliant Inc. (formerly Aliant Inc.) was a separate company providing telecom services in the Atl ...
(NB) and serves
Campobello Island Campobello Island (, also ) is the largest and only inhabited island in Campobello, a civil parish in southwestern New Brunswick, Canada, near the border with Maine, United States. The island's permanent population in 2021 was 949. It is the si ...
,
New Brunswick New Brunswick (french: Nouveau-Brunswick, , locally ) is one of the thirteen provinces and territories of Canada. It is one of the three Maritime provinces and one of the four Atlantic provinces. It is the only province with both English and ...
. Campobello is a border community and is a local call to
Lubec, Maine Lubec ( ) is a town in Washington County, Maine, United States. It is the easternmost municipality in the contiguous U.S. (see extreme points of the United States) and is the country's closest continental location to Africa. The town, with a p ...
, in
area code 207 Area code 207 is the sole telephone area code in the North American Numbering Plan (NANP) for the U.S. state of Maine. Area code 207 was created as one of the original North American area codes in 1947. The numbering plan area still retains its or ...
. The 207-733 prefix serves Lubec, split between
FairPoint Communications FairPoint Communications, Inc. was headquartered in Charlotte, North Carolina, and operated communication services in 31 markets in 17 states, mostly in rural areas. FairPoint services include local and long distance phone service, data, Interne ...
and a pair of competitors using
number pooling Number pooling is a method of reallocating telephony numbering space in the North American Numbering Plan, primarily in growth areas in the United States. Instead of allocating blocks of ten thousand numbers to each carrier in each community, a bl ...
. Both
area code 506 Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while ''surface area'' refers to the area of an open su ...
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 Edmundston is a city in Madawaska County, New Brunswick, Canada. On 1 January 2023, Edmundston will expanded, annexing the village of Rivière-Verte and parts of the local service districts of the parish of Saint-Jacques and the parish of Sai ...
, the geographically-furthest point still in
New Brunswick New Brunswick (french: Nouveau-Brunswick, , locally ) is one of the thirteen provinces and territories of Canada. It is one of the three Maritime provinces and one of the four Atlantic provinces. It is the only province with both English and ...
, the administrator adopts this approach. Similarly, Edmundston is local to
Madawaska, Maine Madawaska is a New England town, town in Aroostook County, Maine, Aroostook County, Maine, United States. The population was 3,867 at the 2020 United States Census, 2020 census. Madawaska is opposite Edmundston, Madawaska County, New Brunswick, M ...
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 South Coffeyville is a town in Nowata County, Oklahoma, United States. As of the 2010 census, the town population was 785. The city of Coffeyville, Kansas is located approximately one mile north of the city, existing as a separate political ent ...
exchange prefixes are served from
Coffeyville, Kansas Coffeyville is a city in southeastern Montgomery County, Kansas, Montgomery County, Kansas, United States, located along the Verdigris River in the state's Southeast Kansas, southeastern region. As of the 2020 United States census, 2020 census, ...
, population 10000; the two are local to each other but long distance to everything else. * The towns of Seekonk and Rehoboth,
Massachusetts Massachusetts (Massachusett language, Massachusett: ''Muhsachuweesut assachusett writing systems, məhswatʃəwiːsət'' English: , ), officially the Commonwealth of Massachusetts, is the most populous U.S. state, state in the New England ...
share a
wire center A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
with
East Providence, Rhode Island East Providence is a city in Providence County, Rhode Island, United States. The population was 47,139 at the 2020 census, making it the fifth-largest city in the state. Geography East Providence is located between the Providence and Seekonk ...
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 Providence is the capital and most populous city of the U.S. state of Rhode Island. One of the oldest cities in New England, it was founded in 1636 by Roger Williams, a Reformed Baptist theologian and religious exile from the Massachusetts Bay ...
metro area with 7 digits. This was phased out when New England Telephone began the process of creating an
overlay area code In telecommunications, an area code overlay complex is a telephone numbering plan that assigns multiple area codes to a geographic numbering plan area (NPA). Area code overlays are implemented in territories of the North American Numbering Pla ...
for the 508 area code. Ten-digit dialing is now required for calls crossing the area code (and state) boundary.


Limitations

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 centers such as
St. Louis St. Louis () is the second-largest city in Missouri, United States. It sits near the confluence of the Mississippi and the Missouri Rivers. In 2020, the city proper had a population of 301,578, while the bi-state metropolitan area, which e ...
or
Kansas City The Kansas City metropolitan area is a bi-state metropolitan area anchored by Kansas City, Missouri. Its 14 counties straddle the border between the U.S. states of Missouri (9 counties) and Kansas (5 counties). With and a population of more ...
(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 __NOTOC__ Year 613 ( DCXIII) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. The denomination 613 for this year has been used since the early medieval period, when the Anno Domini calendar era ...
/
819 __NOTOC__ Year 819 (Roman numerals, DCCCXIX) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar. Events By place Europe * Spring – Emperor Louis the Pious, Louis I marries Judith ...
, 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 Local access and transport area (LATA) is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the Modification of Final Judgment (MFJ) entered by the United States District ...
(LATA) spills into portions of
Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean to ...
and
Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth ar ...
, every number in the area that was in Maryland's
area code 301 __NOTOC__ Year 301 ( CCCI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Postumius and Nepotianus (or, less frequently, year 1054 ...
and northern Virginia's
area code 703 __NOTOC__ Year 703 ( DCCIII) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. The denomination 703 for this year has been used since the early medieval period, when the Anno Domini calendar era ...
was given a "hidden" number in the District's
area code 202 Image:Area code 202.svg, 350px, The red area is the District of Columbia, served by area code 202. poly 0 50 49 54 103 84 111 88 101 107 118 130 143 139 179 140 211 188 238 203 260 240 259 248 248 250 255 262 253 357 1 357 Area codes 571 and 7 ...
. 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 The Eastern Shore of Maryland is a part of the U.S. state of Maryland that lies mostly on the east side of the Chesapeake Bay. Nine counties are normally included in the region. The Eastern Shore is part of the larger Delmarva Peninsula that ...
. Severing the border community from its exhausted home area code using a
split plan Split(s) or The Split may refer to: Places * Split, Croatia, the largest coastal city in Croatia * Split Island, Canada, an island in the Hudson Bay * Split Island, Falkland Islands * Split Island, Fiji, better known as Hạfliua Arts, entertain ...
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 Overlay may refer to: Computers *Overlay network, a computer network which is built on top of another network *Hardware overlay, one type of video overlay that uses memory dedicated to the application *Another term for exec, replacing one process ...
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.


See also

*
Linked numbering scheme A linked numbering scheme (LNS) is a dialing procedure in effect in a service area within which call routing between adjacent exchanges does not require a dialing code. The term is only used in the United Kingdom, but not in the North American ...
*
Area code 202 Image:Area code 202.svg, 350px, The red area is the District of Columbia, served by area code 202. poly 0 50 49 54 103 84 111 88 101 107 118 130 143 139 179 140 211 188 238 203 260 240 259 248 248 250 255 262 253 357 1 357 Area codes 571 and 7 ...
and
301 __NOTOC__ Year 301 (Roman numerals, CCCI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Postumius and Nepotianus (or, less freque ...
/
703 __NOTOC__ Year 703 (Roman numerals, DCCIII) was a common year starting on Monday (link will display the full calendar) of the Julian calendar, the 703rd year of the Common Era (CE) and Anno Domini (AD) designations, the 703rd year of the 1st mi ...
, Washington DC and MD/VA * Area code
314 __NOTOC__ Year 314 ( CCCXIV) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Rufius and Annianus (or, less frequently, year 1067 ''Ab ...
/
618 __NOTOC__ Year 618 ( DCXVIII) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. The denomination 618 for this year has been used since the early medieval period, when the Anno Domini calendar era ...
, St. Louis/East St. Louis * Area code
613 __NOTOC__ Year 613 ( DCXIII) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. The denomination 613 for this year has been used since the early medieval period, when the Anno Domini calendar era ...
/
819 __NOTOC__ Year 819 (Roman numerals, DCCCXIX) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar. Events By place Europe * Spring – Emperor Louis the Pious, Louis I marries Judith ...
, Ottawa-Hull * Area code
712 __NOTOC__ Year 712 ( DCCXII) was a leap year starting on Friday (link will display the full calendar) of the Julian calendar. The denomination 712 for this year has been used since the early medieval period, when the Anno Domini calendar era b ...
/ 605/
402 __NOTOC__ Year 402 (Roman numerals, CDII) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Arcadius and Honorius (or, less frequent ...
,
Sioux City metropolitan area The Sioux City metropolitan statistical area, as defined by the United States Census Bureau, is an area consisting of five counties in three states – Iowa, Nebraska, and South Dakota, anchored by the city of Sioux City, Iowa. As of the 2010 c ...
(three states) * Area code
816 __NOTOC__ Year 816 (Roman numerals, DCCCXVI) was a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar. Events By place Europe * October 5 – King Louis the Pious (son of Charlemagne) is crow ...
/ 913, Kansas City


Notes


References

{{reflist North American Numbering Plan