Advanced History of the North American Numbering Plan - North America Phone Number Plan

Advanced History of the North American Numbering Plan


Why do North American phone numbers have nine digits? Who came up with the numbering system for phones?

The use of seven-digit phone numbers across the United States & Canada started in 1947 through the cooperation of local telephone operators, AT&T, and the Bell System. While the telephone was invented in 1876, it wasn’t actually until 70 years later that the North American Numbering Plan was created.

The numbering system was part of a large-scale effort to improve long-distance calling. It would eventually be referred to as the (NANP) and remains the standard for assigning phone numbers to this day.

Here’s a closer look at the history of the North American Numbering Plan, including why it was necessary and what areas it covers.


Calling a Neighbor Required a Telephone Exchange

Advanced History of the North American Numbering Plan - Calling a Neighbor Required a Telephone Exchange

To understand the importance of the NANP, you need to understand how early telephone systems worked. It all started after Alexander Graham Bell filed the first US patent for a telephone in 1876.

The same year, Tivadar Puskas, a Hungarian engineer, proposed the idea of a telephone exchange to connect pairs of telephones. Before the idea of a telephone exchange, telephones were linked directly with each other. The phones were essentially intercoms.

The American Bell Telephone Company opened the first telephone exchange in 1877. The exchange was opened in Connecticut and had just 21 subscribers, but exchanges were available in almost every major US city within a few years.

The original telephone exchanges provided service for small local areas. The exchanges were operated either manually or automatically using a switchboard using the same “Bell System” introduced in Connecticut.

The American Bell Telephone Company eventually created a subsidiary to handle long-distance calls between Chicago and New York. American Telephone & Telegraph (AT&T) started with just one long-distance line before expanding its network to connect a growing number of cities.

In 1899, the American Bell Telephone Company was restructured. AT&T became the owner of its parent company and all the companies under the Bell System umbrella.

By the early 20th century, the American and Canadian telephone industries included thousands of telephone exchanges and routing centers. Local exchanges were mostly operated by Bell Operating Companies owned by AT&T. In the early 1900s, AT&T began buying up the various local telephone companies that handled local exchanges. The growing monopoly of Bell Systems led to the nickname “Ma Bell.”


Introduction of the General Toll Switching Plan

Advanced History of the North American Numbering Plan - Introduction of the General Toll Switching Plan

By 1930, AT&T had developed the General Toll Switching Plan to provide technical specifications and standards for routing long-distance calls. Switch operators manually forwarded long-distance calls between call centers.

Customers often needed to hang up and wait for the operator to call back after completing the long-distance connection. The average wait time to establish a long-distance call was about 15 minutes. An automatic switching system was introduced in 1943, which decreased the wait time to just two minutes. However, operators were still needed to determine the special routing code for each location.

AT&T hoped to eventually enable direct distance dialing, which would allow customers to directly call long-distance numbers without relying on an operator. To carry out this goal, AT&T needed to develop a consistent numbering plan.


Standardizing Local Phone Numbers

Advanced History of the North American Numbering Plan - Standardizing Local Phone Numbers

At the time, the numbering systems for local telephone numbers varied greatly across North America. Small cities used phone numbers with just two or three digits. Larger cities used phone numbers with up to seven digits.

Telephone exchanges also used unique “central office prefixes.” The prefixes typically contained two or three digits based on the name of the city where the exchange was located. Customers could dial the prefix before the telephone line number to connect to a person in a nearby community.

AT&T slowly standardized the prefixes to avoid duplicating central office names. The new prefixes included two letters and a number. The prefix was followed by the telephone line number, which included four digits. This allowed each prefix to support up to 10,000 telephone lines.


Dividing North America Into Numbering Plan Areas

Advanced History of the North American Numbering Plan - Dividing North America Into Numbering Plan Areas

Along with standardizing local numbers, AT&T needed a way to connect central offices around North America. The company announced plans to divide the continent into 50 to 75 numbering plan areas (NPAs), which are also commonly known as area codes.

The NPAs would allow central offices to easily forward long-distance calls. However, the goal was to allow customers to make long-distance calls without connecting to a toll system. AT&T originally assigned 86 area codes. Alaska and Hawaii did not initially receive area codes. Most states received one area code while a handful received multiple.

New York state was divided into five numbering plan areas. Illinois, Ohio, Pennsylvania, and Texas each received four area codes. California, Iowa, and Michigan each had three. The Canadian province of Ontario and Quebec, along with the US states of Indiana, Kansas, Massachusetts, Minnesota, Missouri, and Wisconsin, each received two area codes.

The NPA system was gradually expanded as states and provinces required additional area codes to support their growing populations.


Why Do Area Codes Include Three Digits?

Advanced History of the North American Numbering Plan -  Why Do Area Codes Include Three Digits?

Each NPA could support up to 540 central office prefixes. As each prefix supported up to 10,000 lines, a single NPA could serve a maximum of 5,400,000 customers.
The most populous states would require more than one NPA. However, multiple NPAs were also needed to support the existing infrastructure of central offices.

AT&T also wanted to continue allowing customers to dial local numbers without adding an NPA. The NPA would only be needed for long-distance calls. This required the company to come up with a way to distinguish between an NPA and a central office code.

As central office codes were based on the names of exchanges, the first two digits could not start with a 0 or a 1. No letters were mapped to the “1” digit on a rotary telephone while only the letter Z was mapped to the “0” digit. Dialing “0” was also the method for reaching an operator or a long-distance exchange.

By making the NPA a three-digit code, AT&T could place a 0 or a 1 in the middle digit, automatically distinguish it from central office codes, and prevent callers from accidentally calling the operator.


AT&T Finally Achieves Direct Distance Dialing (DDD)

Advanced History of the North American Numbering Plan - AT&T Finally Achieves Direct Distance Dialing (DDD)

With the NANP in place, AT&T was ready for the second phase of its plan. It could now start implementing a direct-distance dialing (DDD) system. Before the introduction of DDD, long-distance calls were routed through the nationwide operator toll dialing system. Long-distance operators manually forwarded calls between toll offices.

The DDD system required the use of automatic toll-switching centers, which were originally created in 1943. However, the first customer-dialed long-distance call occurred on November 10th, 1951. A customer placed a direct call from Englewood, New Jersey, to Alameda, California. By the 1960s, direct distance dialing was available in most areas of North America.


Central Office Codes Receive a Major Update

Advanced History of the North American Numbering Plan -Central Office Codes Receive a Major Update

When AT&T standardized central office prefixes in the late 1940s, the company expected the system to remain viable until at least the year 2000. However, by the end of the 1950s, AT&T determined that North America would outgrow the system by around 1975.

The company introduced the All-Number Calling (ANC) system in 1958. With ANC, AT&T eliminated the use of central office names to determine central office codes. For example, the central office in Atlantic City would no longer use the “AT4” prefix. The entire North American network of central offices would receive new codes. However, the central office codes were still restricted from using 0 or 1 in the first two digits.

Reassigning the central office codes increased the potential number of prefixes from 540 to 640. Yet, this was not enough to deal with the increasing populations in major urban areas.


Interchangeable Central Office Codes

Advanced History of the North American Numbering Plan -Interchangeable Central Office Codes

AT&T realized that some urban areas would run out of central office prefixes by 1973. The company finally decided to remove the restriction on the middle digit from including 0 or 1.

The introduction of interchangeable central office codes also required AT&T to change the format for completing long-distance calls. Customers would need to dial “1” before dialing the area code and telephone number. If a customer dialed a “1,” the telephone network would know that the proceeding three digits represented the area code instead of the central office code.

While central office codes were made interchangeable in the 1960s, the NANP Administrator did not make area codes interchangeable until 1995. Area codes can now include any digit for the middle digit, instead of just 0 or 1. However, the first digit cannot include a 0 or a 1.


AT&T Loses Control of the NANP

Advanced History of the North American Numbering Plan -AT&T Loses Control of the NANP

The breakup of Ma Bell led to the end of AT&T as the administrator of the NANP. The US Federal Government had always kept close tabs on AT&T and its Bell System.

In 1913, the government filed an anti-trust suit, which led to the Kingsbury Commitment. Under the agreement, AT&T eliminated Western Union as one of its subsidiaries. In 1934, the Federal Communications Commission (FCC) began regulating AT&T practices. By the early 1980s, AT&T was the largest corporation in the world. The company had assets of $150 billion, which is the equivalent of $370 billion today.

In 1974, the government filed another antitrust lawsuit. It was a lengthy process that extended through the late 1970s and early 1980s. By 1982, AT&T realized that it could not win the lawsuit. AT&T reached a deal with the Justice Department and broke itself up into seven smaller companies. These companies were called “Baby Bells.” Ameritech, Bell Atlantic, BellSouth, NYNEX, Pacific Telesis, Southwestern Bell, and US West were the seven companies which retained control of the phone lines.

Some of the baby bells were eventually reacquired by AT&T or its competitors. For example, American Information Technologies Corporation eventually became Ameritech, which is now a subsidiary of the AT&T holding company.

After the breakup of the Bell System, the FCC assumed control of the NANP. However, the government agency does not directly administer the plan. The FCC solicits contracts from the private sector. Bell Communications Research (Bellcore) became the first administrator following the break-up of AT&T. Bellcore was owned equally by all the Baby Bells.

In 1998, the contract to administer the NANP was awarded to Lockheed Martin. The following year, Neustar, which was a company created by Lockheed Martin to oversee the NANP, received the contract. Somos assumed administrative duties in 2019 and signed a contract the following year to continue administering the plan for another eight years.


What Countries Use the NANP?

Advanced History of the North American Numbering Plan - What Countries Use the NANP?

The NANP was created to provide better telephone services for customers in North America. The initial plan only covered the contiguous United States and Canada. However, other countries eventually joined the plan.

Along with the United States and Canada, most of the countries and territories throughout the Caribbean are members of the NANP and have distinct area codes. This includes popular tourist destinations such as the Bahamas, Bermuda, Jamaica, and Puerto Rico.

The NANP administrators have also discussed the possibility of adding digits. Administrators estimate that the current NANP should work until at least 2049. One plan to deal with a potential expansion involves adding the digits 0 or 1 to the beginning or end of each area code, doubling the number of available area codes.


The NANP Reserves Numbers of Special Use

Advanced History of the North American Numbering Plan - The NANP Reserves Numbers of Special Use

If you dial 0, you can connect to a local operator for assistance. If you dial 00, you connect to a long-distance operator for assistance. These are just a few examples of the dozens of special codes and numbers reserved for specific features or entities.

For example, the following codes cannot be used for area codes or central office codes:

  • 211: Local community information
  • 311: City government or non-emergency police calls
  • 411: Local telephone directory service
  • 511: Road, traffic, and tourist information
  • 611: Telephone line repair service
  • 711: Relay service for callers with hearing or speech disabilities
  • 811: Pipe/cable location services
  • 911: Emergency services

    People over a certain age may also remember *69, which was used to hear the number of the last incoming call. You could dial *69 if you wanted to find out who called you.


    Last Thoughts on the NANP

    Advanced History of the North American Numbering Plan - Last Thoughts on the NANP

    North America still relies on the NANP to allocate telephone numbers. Automatic toll switching and the technologies that led to direct distance dialing are now taken for granted. Most people are unaware that their seven-digit telephone number starts with a three-digit central office code.

    In the coming years, we may even end up with longer codes. However, thanks to smartphones, tablets, and the Internet, no one really needs to remember telephone numbers.




    Stephanie is the Marketing Director at Talkroute and has been featured in Forbes, Inc, and Entrepreneur as a leading authority on business and telecommunications.

    Stephanie is also the chief editor and contributing author for the Talkroute blog helping more than 100k entrepreneurs to start, run, and grow their businesses.

    StephanieAdvanced History of the North American Numbering Plan