When describing hos telephone numbers are constructed in a country, you have the open or closed number plan terms:
| Number plan type | Example | Description |
|---|---|---|
| Open | Sweden | Distinguishes between how to dial local calls within an area code area or outside it. If you are calling within Uppsala, you do not have to dial 018. If you are calling from Stockholm to Uppsala you have to dial 018. In countries with an open number plan, the length of the telephone numbers often varies. In Uppsala you find e.g. local numbers with six or seven digits. |
| Closed | Norway | In some countries the traditional area code numbers have been skipped and this information is instead included in all telephone numbers. A closed number plan is used in Norway, and if you are calling a telephone in Oslo you use the same number, regardless of whether you are in Olso or in Bergen. All telephone numbers are mostly of the same length and there can be a geographical distinction between the numbers. |
To describe the format for separate telephone numbers, the following terms are used:
| Number type | Example | Description |
|---|---|---|
| International format | +4618182900 | Provides enough information to make a call, even between two countries. Always contains the + character, country code, any area code number without area code number prefix, and a local number. |
| National format | 018182900 | Can be dialled within the country the number is intended for, but not from any other country. Always contains any area code number prefix, any area code number, and a local number. |
| Local number/ Subscriber number | 182900 | If an open number plan is applied, a local number only functions within an area code area.
Without location information, it is difficult to distinguish a local number from an extension number. In countries with closed number plan there is no difference between national format and subscriber number. |
| Extension number (DN) | 2900 | Used within a PBX. Only functions to make a call from another extension in the same PBX. Is not a complete local number.
Without information about location, as described below it is difficult to distinguish an extension number from a local number. |