From aa-asterisk.org.uk wiki
E.123 is the ITU-T standard for Notation for national and international telephone numbers, e-mail addresses and Web addresses.
Telephone numbers usually comprise:
- an international country code (shown only in international format)
- an area code, which may be optional for some callers to dial
- a local subscriber number.
It is customary to show each of those elements as separate groups with a space separator between each group.
Some countries do not use area codes. Those are places with a closed numbering plan.
- only spaces should be used to visually separate groups of numbers "unless an agreed upon explicit symbol (e.g. hyphen) is necessary for procedural purposes"
- parentheses are used to indicate digits that are sometimes not dialled.
- only spaces should be used to visually separate groups of numbers
- parentheses should not be used in the international notation
- digits that international callers do not dial (such as "0" trunk code) should be omitted.
Additionally, a solidus, or slash, (/) may be used to indicate alternate numbers. (i.e. "7222 1234/7890" means 7222 1234 and 7222 7890).
|Telephone number, national notation||(020) 7222 1234|
|Telephone number, international notation||+44 20 7222 1234|
|Web address / URL||www.example.com|