Telephone numbering plan

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"Area Code" redirects here. For the song by Ludacris, see Area Codes (song).

A telephone numbering plan is a type of numbering scheme used in telecommunications to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined in each of administrative regions of the public switched telephone network (PSTN) and they are also present in private telephone networks.


The North American Numbering Plan is a closed numbering plan and prescribes ten digits for each complete destination routing code. A complete telephone number consists of three basic parts. The most significant part is a 3-digit Numbering Plan Area (NPA) code (area code). Each area code comprises a set of 3-digit central office (CO) codes, which are unique to each telephone exchange within an NPA. The remaining four digits are the specific station number assigned to each telephone.




Initial use of area codes in the United States and Canada began in 1947 in large cities.[1] By 1966, the system was implemented fully in both countries.[2] Area codes were initially assigned based on the volume of telephone calls made in each area. The most populous areas were assigned the area codes that required the least time for dialing using a rotary dial telephone. A subscriber line as well as switch banks had to be allocated for the entire time from begin of dialing to the end of a call. The time to dial a digit was directly proportional to the digit, with the exception of the 0, which required 10 pulses or 1 second. The densely populated areas of New York City, Chicago, Los Angeles, and Detroit had large incoming call volume and were assigned the shortest area codes (212, 312, 213, and 313, respectively). A sparsely populated area of rural Texas received area code 915. Area codes which covered an entire province or state received the less-desirable '0' middle digit. The first area code installed was 201 for New Jersey. Such numbering strategies became less important when the Bell System introduced Dual-tone multi-frequency (DTMF) signaling in its Touch-Tone service starting in 1963, where all digits were generated in the same amount of time.

When the North American Numbering Plan was developed, telephone circuits included electromechanical relays which imposed limitation on the speed of dialing a digit, and therefore dialing a complete 10-digit telephone number could take up to ten seconds, during which hardware resources had to be dedicated to a circuit. The Bell System organized the numbering plan to minimize the cost of providing automatic dialing to large population centers. The second digit of all area codes was 0 or 1, while the second digit of the exchange triplet was never 0 or 1, thus facilitating the recognition of whether a user was dialing a full 10-digit number or merely dialing within the local area code. In this coding scheme it was not yet necessary to dial a leading 1 for access to long distance circuits to other area codes. In some regions, local numbers were 7 digits, or occasionally less, but a toll call within the area code required a preceding 1. Area code 617 in eastern Massachusetts used this system in the early 1970s, while the Chicago area (312) did not. In the Chicago area, one could call the Boston area by dialing only 10 digits, while in Boston, to call Chicago, one would be required to use 11 digits, including the prefix 1.


Because of the popularity of fax machines and cell phones, far more telephone numbers were needed to handle the demand, and the decision was taken to add more area codes rather than increase the number length. Since 1995, over 380 new area codes were added to the North American Numbering Plan. Some areas used area code splits, by which an existing area code was split up into multiple new area codes. This cost businesses and others a lot of money reprinting stationery, business cards, etc. These were often greatly contested as to which area could keep the old area code. Usually the largest city kept the existing code. For instance, 305 was split in 1995. It used to serve both the Miami and Fort Lauderdale area. Dade County (today called Miami-Dade) kept 305 and Broward County (the Fort Lauderdale area) had to change to 954. Because splits were challenged so strongly, overlays were created when possible. An overlay is a new area code that covers the same geographical area as an existing code. This enabled subscribers to keep their old area code, while new phone numbers would get the new area code. Over 75 overlays have been introduced since 1995.[3]

Currently, because of area code overlays, nearly all metropolitan and many rural telephone calls require the full 10 digit entry to complete a telephone call.



The E.164 numbering plan for telephone numbers includes:


Country code[edit]


Area code[edit]

Numbering plans for geographically allocated telephone numbers typically designate an area code as the prefix routing code for a region. Area codes are defined differently by the various telecommunication administrations.





When the Bell System first designed the area codes concept, the initial 3-digit number format was NBX, where N could be any digit from 2 to 9, B either 0 or 1, and X any digit. Since all telephones in this era used pulse dialing mechanisms, the engineers sought to reduce the time required for dialing during which a circuit had to be dedicated, as well as lower the overall mechanical wear by reducing the number of clicks on numbers that are dialed most often. Areas with higher call volume would be assigned lower first and third digits and a 1 as the center digit. The digit 0 was located at the end of the dial after 9, making zero the longest digit to dial. New York, having the highest call volume, was assigned the 212 area code, the shortest code that could be dialed. This was followed by Los Angeles at 213, and Chicago at 312. The next code, 313, designated Detroit, because it had a larger call volume than Philadelphia, the next largest in population at the time. Philadelphia received the area code 215, while the Dallas–Fort Worth area received 214, even though Dallas–Fort Worth was less populous at the time.


Subscriber number[edit]


Area code and call pricing[edit]

In countries other than the United States and Canada, the area codes generally determine the cost of a call. Calls within an area code and often a small group of adjacent or overlapping area codes are normally charged at a lower rate than outside the area code. This is not necessarily the case in the United States or Canada where rates are determined by the distance between rate centers. In most United States and Canadian locations area codes cover a sufficiently large territory that different rates will apply within the same area code determined by the distance between rate centers. Each rate center also has a local calling plan which determines which other rate centers, regardless of distance, are a local call. The area code plus the first three digits following the area code (the NPA-NXX) defines the rate center for any given telephone number. Multiple NPA-NXX combinations, often in different area codes, comprise each rate center, and each rate center is assigned a geographic point known as the rate center's V&H coordinates. The tariff distance in miles between any two rate centers may be calculated using this formula: \sqrt { \frac{(V1-V2)^2 + (H1-H2)^2} {10} }.




Customer dialing procedures[edit]




Variable-length dialing[edit]


To call a number in Sydney, Australia for example:




  • 1 306 xxx xxxx (within the 306 area code, but not within the Regina local calling area (e.g., Saskatoon))
  • 1 306 xxx xxxx (anywhere within the NANP outside the 306 area code)
  • +1 306 xxx xxxx (outside NANP)

To call a number in San Francisco, California the dialing procedure varies:

  • xxx xxxx (local or long-distance within area code 415, no area code required)
  • 1 415 xxx xxxx (local or long-distance outside of 415 but within the U.S., Canada, and other countries in the NANP)
  • +1 415 xxx xxxx (outside the NANP - 1 is the country code for the U.S.)



  • +1 610 xxx xxxx (outside the NANP)



Full-number dialing[edit]


  • xxx xxx (within Oslo - no area code required)
  • +47 2 xxx xxx (outside Norway)


  • 22xx xxxx (within Norway - including Oslo)
  • +47 22xx xxxx (outside Norway)




  • Rome 06 xxxxxxxx (outside Italy +39 06 xxxxxxxx)



  • 2xx xxx xxx (inside Czech Republic)
  • +420 2xx xxx xxx (outside Czech Republic)

as opposed to before September 21, 2002:[8]

  • 02 / xx xx xx xx (inside Czech Republic)


  • xxx-xxxx (inside Bangkok)
  • 02-xxx-xxxx (inside Thailand)
  • +66 2-xxx-xxxx (outside Thailand)

has been switched in 1997:

  • 2-xxx-xxxx (inside Thailand)
  • +66 2-xxx-xxxx (outside Thailand)

Trunk prefix has re-added in 2001

  • 02-xxx-xxxx (inside Thailand)
  • +66 2-xxx-xxxx (outside Thailand)

International numbering plan[edit]



Satellite telephone systems[edit]


  • ICO Global: +881 0, +881 1.
  • Ellipso: +881 2, +881 3.
  • Iridium: +881 6, +881 7.
  • Globalstar: +881 8, +881 9.
  • Emsat: +882 13.
  • Thuraya: +882 16.
  • ACeS: +882 20.

+ 88184

Special services[edit]


Numbering plan indicator[edit]


NPI Description Standard
0 unknown
1 ISDN Telephony E.164
2 generic
3 data X.121
4 telex F69
5 maritime mobile E.210 and E.211
6 land mobile E.212
7 ISDN/mobile E.214

Private numbering plan[edit]







See also[edit]


  1. ^ Area Code History
  2. ^
  3. ^ " and". Retrieved 5 February 2013. 
  4. ^ Twentieth Century Hong Kong Dr. Hans YEUNG. References and Resources of History subject, Education Bureau, Hong Kong. (5) for Hong Kong Island, (3) for Kowloon, (0) for New Territories.
  5. ^ Saunders, Amy (2009-05-16). "Cell-phone age turns the 614 into just numbers". The Columbus Dispatch. Retrieved 2009-08-21. 
  7. ^ 2010 Otago White Pages. Yellow Pages Group. pp. 8, 80, 177. 
  8. ^ "Číslovací plán veřejných telefonních sítí" (PDF). Telekomunikační věstník (in Czech) (Czech Telecommunication Office). 9/2000. 2000-09-25. Retrieved 2006-10-13.  (in Czech language)
    Numbering Plan for Public Telephone Networks - annotation of the article in English[dead link]
  9. ^ Contacting Saarland University

External links[edit]