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Numbering Plan Areas (NPA)

NPA, also known as an Area Code or a number planning area/numbering plan area/numbering plan area, is the three-digit code that identifies the phone service region. For example, if you have the telephone number 555-101-1234, your NPA would be 555.

Although the Area Code identifies the telephone service area, one NPA can be present at more than one rate center. For example, the 305 area code is found in Florida’s rate centers Keys, Miami, Homestead, and North Dade and Perrine. Therefore, you can order a number from NPA 305 to get a phone number at any one of these rate centers.

NPA Background

Number Plan Areas can be based on national, regional, or global phone numbering plans. These areas serve as guidelines that dictate how phone numbers should look. These numbering plans can be used for both public and private networks. Telephone numbering plans are often used to assign numbers to public telephone networks based on geographical locations – hence the ‘area in the number plan area.

Numbering Plan Areas
Numbering Plan Areas

Number Plan Area (NPA)

A Number Plan Area (NPA) is a telephone service territory that has been established under the guidelines set out in the relevant telephone numbers plan. Telephone numbering plans use NPAs to delineate service territories based on their geographical location. Every NPA is assigned an area code (number plan area code), whose length is determined according to the telephone numbering plans. This is the abbreviation of the long-winded “number plan area code.”

Bell System engineers devised the NPA concept in the early 1940s as part of a plan to replace operator toll dialing with direct subscriber-to-subscriber calling. The NANP divides the service territories into Number Plan Areas. Codes attach to each phone number issued within the NPA’s service territory.


The History of the Number Plan Area NPA

Engineers at Bell System are responsible for creating the Number Plan Area concept. Ma Bell created the idea as part of a larger plan to replace dialing technology that relied on human operators to route calls. Unfortunately, its dependence on human operators severely limited the system’s growth. To solve the problem, Bell System engineers decided to take human operators out of the equation by automating the system to make direct subscriber-to-subscriber calls possible.

One part of the solution was to create a standard way to assign phone numbers to subscribers. This would make it easier to dial and route calls. It divided North America into 86 different Numbering Plan Areas and assigned each a three-digit number. It was common for forms with multiple area codes to get NPA codes. For those with only one, it was a zero. This allowed for more growth over the following decades by leaving out higher numbers than one in the second number of these first area codes. For example, the first digit could contain any number between 2 and 9, while the last could have 0 and 9. 

Other factors that could have determined which areas got which codes were population and how easy it was to dial a code on a rotary telephone. Area codes required fewer clicks to assign more significant people with high call volumes, such as New York City and Los Angeles. In addition, the NANP includes Sint Maarten and Saint Kitts and Nevis and Saint Lucia, Saint Vincent and the Grenadines, and Trinidad and Tobago.

Initially, the NANP was administered by AT&T. This organization doesn’t make policy. Instead, its role is to follow industry standards and policy directives. Each country in the plan is responsible for managing the number of resources within their respective jurisdictions.

Numbering Plan Areas
Numbering Plan Areas

The Number Plan Area: Mechanics

As we have seen, the NANP divides telephone service territories into numbering plan areas (or NPAs) and assigns each an area code of three digits. This code is the first part of telephone numbers. The remainder consists of a three-digit central office (CO), a prefix that indicates a particular exchange or rate center, and a four-digit station number. The destination routing address for the public switched telephone network is the combination of the area code, the office code, and the station number. The international calling code for the plan is 1. In addition, the NANP is compliant with the International Telecommunications Union phone numbering plan known as E.164.

Callers don’t have to dial an area code if the recipient is in the same NPA. Automatic telephony devices, however, tend to dial the whole number, including area codes. see also avaya.

Nearly 800 CO codes or prefixes are available for each area code. Some, such as 911, are not usable because they are public. Each prefix contains 10,000 telephone numbers. Every area code has nearly eight million telephone numbers. Although this sounds like a lot, it is not. Some metros go through them in just a few decades because of population growth, economic progress, and increased demand for cellular services. They replace once exhausted as it is impossible to reassign codes.

Another solution is overlaying an existing NPA to add area code. This is a common approach. This minimizes service interruptions, but it can also mean that adjacent businesses might have different area codes. This situation requires a bit more dialing.

Types of Area Codes for Number Plan

Fixed Length NPA Codes

NPA code prescribes by a telephone numbering plan to be unvarying in length. The length of NANP NPA codes is three digits. Australia’s telephone number plan restricts NPA codes to one digit.

Variable Length NPA Codes

Many countries have telephone numbering plans that use variable-length NPA codes. These include Japan also Germany, which have area codes ranging from one to five digits in length. The United Kingdom and Germany.

Recognizable Codes

Easily Recognizable codes (ERCs), or N11, are NANP area codes whose second and last digits are similar. These codes are easy to remember, such as 911 and 411. Know more about Are There Uptime Issues with a Virtual Phone System? & MCC Billing.