How Telephone Country Codes Work

The Public Switched Telephone Network (PSTN), also known as Plain Old Telephone Service (POTS), is the wired phone system over which landline telephone calls are made. The PSTN relies on circuit switching. To connect one phone to another, the phone call is routed through numerous switches operating on a local, regional, national or international level. The connection established between the two phones is called a circuit.

In the early days, phone calls traveled as analog signals across copper wire. Every phone call needed its own dedicated copper wire connecting the two phones. That's why you needed operators' assistance in making calls. The operators sat at a switchboard, literally connecting one piece of copper wire to another so that the call could travel across town or across the country. Long-distance calls were comparatively expensive, because you were renting the use of a very long piece of copper wire every time you made a call.

Beginning in the 1960s, voice calls began to be digitized and manual switching was replaced by automated electronic switching [source: WirelessCenter]. Digital voice signals can share the same wire with many other phone calls. The advent of fiber-optic cables now allows thousand of calls to share the same line. But fiber-optic and other high-bandwidth cables haven't changed the basic nature of circuit switching, which still requires a connection -- or circuit -- to remain open for the length of the phone call.

Routing calls requires multiple switching offices. The phone number itself is a coded map for routing the call. In the United States, for example, we have 10-digit phone numbers.

Within a company or larger organization, each employee or department might have its own extension. Extensions from the main phone number are routed through something called a private branch exchange (PBX) that operates on the premises.

To make an international call requires further instructions. The call needs to be routed through your long-distance phone carrier to another country's long-distance phone carrier. To signal such a switch, you have to dial two separate numbers, your country's exit code (or international access code) and the corresponding country code of the place you're calling.

Almost all exit codes are either 00 or 011, although there are a few exceptions like Cuba (119) and Nigeria (009). Country codes are one- to three-digit prefixes that are assigned to specific countries or groups of countries. For example, the country code for the United States is 1, but the United States shares that country code with Canada and several smaller island nations like Jamaica, Puerto Rico and Guam. The country code for Mexico is 52 and Saudi Arabia is 966. Here's a full list of exit and country codes.

Now let's find out who sets the standards by which the international telephone system operates.

The Telecommunication Standardization Sector is part of the International Telecommunication Union (ITU), an agency of the United Nations. The ITU, based in Geneva, Switzerland, works with 191 member countries to develop and implement global communications technology.

The specific responsibility of the Telecommunication Standardization Sector (ITU-T) is to research and recommend standards and protocols relating to voice and data transmissions over landline and mobile networks. This includes everything from streaming video on cell phones to Voice over IP (VoIP) to SMS to international call rates.

When it was established in 1925, the  ITU-T was called the Comité Consultatif International Téléphonique et Télégraphique (CCITT). The CCITT was responsible for breakthrough work in standardizing fax data transmissions, modems, data compression, packet-switching and e-mail. The CCITT became the ITU-T in 1993 as part of a new strategic plan to respond quicker to the ever-changing technological landscape [source: International Telecommunication Union].

The ITU-T is best known for its recommendations. As their name implies, recommendations aren't laws or regulated standards, but merely suggestions for the best way to make telecommunications technology and networks run smoothly. Recommendations are established by 13 study groups within the ITU-T. Each study group is comprised of international managers and rapporteurs (appointees) from the public and private sector. A typical study group might include a CTO from Israel, a researcher from Germany and an American policymaker from the Federal Communications Commission.

Study groups are assigned a handful of questions, which they research over the course of two to four years before publishing their recommendations. Questions resemble topics. Current examples of questions include:

The ITU-T also organizes focus groups, which are smaller research units within a study group working on a specific problem or question. Recommendations are published online and are free to the public as well as private industry and government agencies.

Next let's talk about the ITU-T recommendation that established the current international numbering plan.

The E.164 recommendation, also called the "international public telecommunications numbering plan," was first approved and published by the ITU-T in May 1997. The E.164 recommendation establishes a standard framework for every country to create its own international phone numbers.

An international E.164 number is designed to include all of the necessary information to successfully route a call to an individual subscriber on a nation's public telephone network. Here's how the E.164 numbering plan works:

The combined length of the national (significant) number can't exceed (15-n), where n is the amount of digits in the country code. This allows each country to decide how many digits should be in the national destination code and the subscriber number. A country with a relatively small population and few major cities, for example, might choose to have fewer digits in their phone numbers. And for larger countries, the possibilities are nearly endless. A 15-digit number allows for 100 trillion different permutations, enough for each person on earth to have 10,000 phone numbers [source: SearchNetworking].

The United States subscribes to a system called the North American Numbering Plan. The North American Numbering Plan was actually created by AT&T in 1947, but it conforms with the framework recommended by the E.164 [source: North American Numbering Plan]. The North American Numbering Plan has a one-digit country code, a three-digit national destination code (called a Numbering Plan Area code, or just area code) followed by a seven-digit subscriber number.

Not all nations have a standard amount of digits for every location in the country. In Mexico, for example, the three largest cities -- Mexico City, Guadalajara and Monterrey -- have eight-digit subscriber numbers while all other locations have seven-digit numbers. The important thing is that all Mexican phone numbers adhere to the standards set forth by the E.164 recommendation.

The E.164 recommendation is currently being expanded into a much broader protocol called ENUM, short for TElephone NUmber Mapping. Using the international E.164 number as a model, ENUM will assign a specific Uniform Resource Identifier (URI) to each and every networked device, including analog telephones and fax machines, mobile phones, computers and PDAs. With this new URI, all these devices will be able to contact each other directly using a single network address/phone number. As of this writing, a final ENUM recommendation hasn't been published.

For lots more information about telephone country codes and related topics, check out the links on the next page.