Telephone Numbering Plan - Standards

Standards

Although the International Telecommunication Union (ITU) has attempted to promote common standards among nation states, numbering plans take different formats in different parts of the world. For example, the ITU recommends that member states adopt 00 as their international access code. However, as these recommendations are not binding on member states, some have not, such as the United States, Canada, and other countries and territories participating in the North American Numbering Plan.

The international numbering plan establishes country codes, that is, area codes that denote nations or groups of nations. The E.164 standard regulates country codes at the international level and sets a maximum length limit on a full international phone number (15 digits). However, it is each country's responsibility to define the numbering within its own network. As a result, regional area codes may be:

• A fixed length, e.g., three digits in the United States and Canada; two digits in Brazil; one digit in Australia and New Zealand,
• A variable length, e.g., between 2 and 5 in Germany, Argentina, United Kingdom and in Austria; between 1 and 5 in Japan; 1 or 2 in Syria and Peru, or
• Incorporated into the subscriber's number, as is the case in many countries, such as Spain or Norway. This is known as a "closed" telephone numbering plan. In some cases a trunk code (usually 0) must still be dialed, as in Belgium, Italy, Switzerland, South Africa and some locations within the NANP. Hong Kong used to have 1-digit area codes. They were incorporated into the subscriber's numbers back into the 1990s. No trunk code is required.