the man dials the phone numbers close upApril 22, 2020

What You Should Know About The NANP

To keep up with the demands of the fast-expanding telephone network in the United States in the middle of the 20th century, the Bell System led by the Bell Telephone Company introduced the North American Numbering Plan (NANP), a telephone numbering plan designed to replace human operators who manually routed calls to their destinations. Today, the North American Numbering Plan serves twenty North American countries, including the United States and its territories. Here is everything you need to know about the NANP.

History Of The NANP

To understand the logic behind the present-day phone numbering scheme and area code number origin, we need to start from the beginning. The North American Numbering Plan came into existence in the 1940s. The purpose of the newly created numbering plan was to change the way the system operated up to that point.

To replace the complex manually-operated system, the Bell System introduced the North American Numbering Plan as a new way of putting calls through. The new automated system enabled anyone to contact whomever they wanted anywhere on the continent. Northwestern Bell originally advertised the new dialing system as an advantage for the operators, stating that operators working long-distance calls would be able to easily and directly dial calls to telephones on the other end of the continent. In reality, the truth was much different. The plan was to eliminate operators altogether since it would be impossible to employ a sufficient number of workers to satisfy future demand. 

With this in mind, Bell came up with the Numbering Plan Areas consisting of 86 separate areas across North America, but the main problem was how to efficiently organize the system. Instead of focusing on the first digit of a three-digit area code and geographical location, the accent was on the second digit and the determining factor was the population count. Each assigned area code number had either a 0 or a 1 as a second digit. Depending on how valuable the state was, it was assigned a 1 or a 0, with 1 being much easier to dial. For instance, California got the area codes 916, 415, and 213, Chicago got 312, and Detroit got 313, which took the least dialing on the analog dial.   

Undoubtedly, the best area codes were given to the largest cities at the time, but the decision was prompted by an estimated future growth. By not assigning any numbers higher than 1 on the second digit, the North American Numbering Plan creators left plenty of room for future expansion of area codes. Other than that, there was no other rule on how area codes were distributed, except for trying to refrain from putting similar area codes right next to each other as was the case with 703 for Virginia and 704 for North Carolina that were later separated.

How Does The NANP Work

The idea behind the North American Numbering Plan was to create a system that could enable callers to reach people on the other side of the telephone wire without the operator’s assistance. The plan was devised in such a manner that it required very few changes to the existing telephone system, which only took four or five and somewhere even fewer digits to dial. 

Originally, the Numbering Plan Area or NPA was limited to the US and Canada. States and provinces were divided into numerous areas and assigned a one-of-a-kind three-digit code that preceded local telephone numbers. The phone exchanges were still in place and turned into local exchange points in the nationwide system along with central offices. The NPA prefix combined with the central office code was used as a destination routing code for any other central office in the system. As a result, each NPA was restricted to 540 central offices.

The most populated states were separated into several NPAs, hence the state of New York received five NPAs, Texas four, California three, and so on. Telephone subscribers were issued four-digit numbers, which meant that each telephone number consisted of the area code, central office code, and line number, adding up to ten digits in total. The purpose of this plan was to enable subscribers to dial a seven-digit number instead of a ten-digit number when placing local calls inside their plan area. 

After the breakup of the Bell System in 1982, the Federal Communications Commission became the administrator of the North American Numbering Plan. In the late 80s, the NANPA introduced a new calling procedure toward long-distance dialing where all calls within an area code must be preceded with the area code for it to be possible to have either a 0 or a 1 as second digits within central-office prefixes, the exception being the N11. This new method allowed for more number combinations and even made it possible to include numbers other than 0 and 1 as the second digit. 

As of yet, The NANP does not cover all North American countries (Mexico, Greenland, Saint Pierre, and Miquelon). Central American countries including El Salvador, Costa Rica, Belize, Guatemala, Honduras, Nicaragua, and Panama and Caribbean countries including Cuba, Haiti, and the French Caribbean do not participate in the NANP either. 

To clarify, the NANP number format looks like this: NPA-NXX-XXXX.

  • NPA – The Numbering Plan Area Code has three digits. The first digit allows numbers from 2-9 while the second and third digits allow numbers from 0-9. The numbers containing the last two identical numbers are known as easily recognizable codes or ERC. ERCs are assigned to special services, for instance, 888 is used for toll-free services, 500 numbers are used for directory assistance and personal 500 numbers, 700 are carrier-specific numbers, and there are high-toll 900 numbers.
  • NXX – The Central Office Code. The first digit allows numbers from 2-9 while the second and third digits allow numbers from 0-9, except in the case when both the second and third digits are 1.
  • XXXX – The unique number assigned to the subscriber. It can contain numbers from 0-9 for each of the four digits. 

The international format of a NANP number must contain the country calling code 1 following the NPA-NXX-XXXX scheme. Even though the national numbering plan of the NANP was intended to contain up to ten digits, many adjustments were made to reconstruct switching systems to support a seven-to-twelve digit format.

NANP Expansion

Due to the accelerated growth of the number of area codes in the US and Canada at the end of the 20th century, there was an increased demand for telephone numbers. To meet the requirements, area codes were assigned in two ways: number plan area splits and overlay plans. 

  • Splitting a number plan area involves dividing one area into several regions. One of the regions gets to keep the existing area code while the other(s) receives new ones. 
  • An overlay plan has multiple codes assigned to one geographical area.

There have been some variations to these methods, including dedicated overlays with new codes reserved for a distinct use like pager or cell services and concentrated overlays where a part of the area keeps the existing area code while other parts get assigned an overlay code. The main difference between a split and an overlay plan is that with the overplay plan, the area code must be dialed before all calls, even local ones, while the split plan requires only seven digits inside the same area. 

Area Codes By Countries & Territories

The number of area codes per state depends on the state. For instance, the state of California has the most assigned area codes. Other states with multiple area codes include Texas, Florida, and New York. The majority of Caribbean countries use only one area code while the country code is based on alphabetic abbreviations of the country’s name. What follows is an overview of area codes according to country and territory.

  • Anguilla: 264, letter code ANG
  • Antigua and Barbuda: ANT
  • Bahamas: BHA
  • Barbados: BIM
  • Bermuda: 441
  • British Virgin Islands: 284, letter code BVI
  • Canada: multiple area codes 204, … 905
  • Cayman Islands: 345
  • Dominica: 767, letter code ROS
  • Dominican Republic: 809/829/849
  • Grenada: 473, letter code GRE
  • Jamaica: 876/658
  • Montserrat: 664
  • Saint Kitts and Nevis: 869
  • Saint Lucia: 758, letter code SLU
  • Saint Vincent and the Grenadines: 784, letter code SVG
  • Sint Maarten: 721
  • Trinidad and Tobago: 868, letter code TNT
  • Turks & Caicos: 649
  • The United States of America: multiple area codes 201, … 989
  • US Territories:
    • American Samoa: 684
    • Guam: 671
    • Northern Mariana Islands: 670
    • Puerto Rico: 787/939, letter code PUR
    • US Virgin Islands: 340

Other Numbers & Codes

Other than the numbering plan area code and the central office code, there are other numbers and codes reserved for special services provided on a nationwide level.


  • 0 – operator
  • 00 – long-distance operator
  • 01 – international access code through operator for destinations outside the NANP
  • 011 – international access code with direct dialing for destinations outside the NANP
  • 211 – social service
  • 311 – non-urgent police matter or city employees
  • 411 – local directory assistance
  • 511 – traffic, road, and tourism info
  • 611 – phone repairment service and wireless provider customer care
  • 711 – call line for customers with hearing and speech deprivation
  • 811 – dig safe pipe/cable location and non-emergency telehealth service
  • 911 – police, firemen, ambulance

Star Codes

  • *51 and 1151 – missed calls for a particular number
  • *57 and 1157 – call trace
  • *66 and 1166 – retrying busy line
  • *67 and 1167 – caller ID block
  • *69 and 1169 – call return
  • *70 and 1170 – cancel waiting for call option for every call separately
  • *71 and 1171 – three-way calling
  • *74 and 1174 – speed dial
  • *75 – allows up to 30 speed-call numbers with two digits
  • *77 – unknown call rejection
  • *82 and 1182 – caller ID blocking per call
  • *87 – deactivation of the anonymous call rejection service

Mobile Phones

The North American Numbering Plan neither requires separate area codes for cell phone numbers nor provides special non-geographic area codes for the use of mobile phones. The NANP does, however, assign different central office prefixes to each area code for cell phone numbers and the calls cost the same as for other types of calls.

Unlike in other countries where a caller is charged a higher rate for calls made to mobile numbers, the North American cellular telephone subscribers are charged for the calls they receive. Nevertheless, to compete with each other, network carriers have reduced the cost of calls for customers with whom they have contracts to match prices in countries where charges are made to the caller. Four of the biggest national carriers in the US, AT&T, Sprint, T-Mobile, and Verizon, offer free minutes between cell numbers on the same network and Sprint even provides calls to other networks without charges. 

As the population continues to grow, so does the demand for additional area codes. What was once an area code for an entire state is today barely enough for one major city. Despite all of its efforts, it is highly unlikely that the NANPA will be able to replace the existing system and come up with a better one. While the plan for the future remains to be unveiled, chances are the NANPA will add another digit or two to the ten-digit number format to meet the ever-increasing need for more and more numbers.