Telephone numbering plan

A telephone numbering plan is a system used in telecommunications networks to give out telephone numbers to phones and other devices that can make calls. These numbers work like addresses, helping calls find their way to the right place.

In most places, where you live can affect your phone number. Areas are often divided into regions with special codes, called area codes or city codes, which are the first part of the number you dial. Inside these areas, phone numbers are chosen based on local rules but still follow bigger network guidelines.

There are different ways to design these numbering plans. Some, like in North America, have fixed-length area codes and local numbers. Others, mostly in Europe, allow the length of area codes and local numbers to vary. The International Telecommunication Union created a global standard called E.164, which sets a maximum of 15 digits for any phone number and gives each country a unique country code to use when calling internationally.

Big companies and organizations sometimes have their own private numbering plans for phones inside their buildings, managed by a central system called a private branch exchange (PBX). These plans decide how people should dial numbers to reach others, which can sometimes be simpler than public phone systems.

Telephone number structure

National or regional groups that help with phone services use special plans to organize phone numbers. These plans follow a global rule called E.164. This rule says a phone number has a country code and a national number.

Countries can have their own ways of making phone numbers. For example, in the United States, all phone numbers have ten digits. In some European countries, the number of digits can change. Phone numbers can tell where someone is, like a town or a phone office. Area codes are special groups of numbers that show a place. In the United States, area codes have three digits, but other countries use different lengths.

International numbering plan

Recommendation E.164 by the International Telecommunication Union sets up an international system for making phone calls between countries. It gives each country a special code, called a telephone country code, so calls can be routed correctly. These codes are added when you dial a number from another country. The full international phone number can have up to 15 digits, including the country code, area code, and the person's number.

Some special codes are used for things like free phone numbers, satellite phones, and disaster help. For example, satellite phones might use codes like 870 or 881. In some cases, several countries share one country code, like the North American Numbering Plan where many places in North America and the Caribbean all use the code 1.

Private numbering plan

Just like big phone systems, smaller phone networks inside places like offices, schools, or hotels can have their own special way of giving out phone numbers. These special numbers are often called extension numbers. People inside the network only need to dial these smaller numbers to talk to each other.

These extension numbers can be set up in handy ways. For example, a hotel might use room numbers as extension numbers, or a hospital might use numbers that spell out words like “help” on a phone pad. Sometimes, these numbers can also be called from outside the building using a special format that adds the extension number to the main phone number.

Numbering plan indicator

In telephone networks, there are special codes called numbering plan indicators (NPI). These codes help the network understand what kind of number it is dealing with, like whether it's a regular phone number or something else. NPIs are part of messages that tell the network how to connect calls and are used in services like short message service.

The codes are defined by international standards, and as of 2004, several different numbering plans and their NPI values were already set.

Subscriber dialing procedures

A telephone numbering plan tells us how phone numbers are set up, but it also includes rules about how to dial numbers. These rules help us know which numbers to dial for local or long-distance calls, and they can include special short numbers for emergencies.

In places like North America, sometimes you only need to dial part of a phone number for local calls, but for longer distances, you need the full number, including an area code. For example, in some areas, you might just dial seven digits for a local call, but for a call outside your area, you need to add the area code. In other countries, you often need to add a special number before the area code when calling within the same country.

When calling internationally, you usually need to add a country code before the area code and number. For example, to call Sydney, Australia, from another country, you might dial the international access code, then the country code, then the area code and number. Some phones let you use a plus sign (+) instead of the access code. These rules help make sure calls go to the right place, no matter where you are.