ITU-T Recommendation E.123, or the Notation for national and international telephone numbers Recommendation E.123 defines a standard way to write telephone numbers, e-mail addresses, and web addresses. It recommends the following formats (when dialling the area code is optional for local calling):
Telephone number, national notation: (042) 123 4567
Telephone number, international notation: +31 42 123 4567
E-mail address: name@example.com
Internet address / URL: www.example.com
E.123 also recommends that a hyphen (-), space ( ), or period (.) be used to visually separate groups of numbers. The parentheses are used to indicate digits that are sometimes not dialled. Parentheses should not be used in the international notation. A slash (/) is used to indicate alternate numbers.
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