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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 recommends that only spaces be used to visually separate groups of numbers, "...unless an agreed upon explicit symbol (e.g. hyphen) is necessary for procedural purposes..." in national notation. Only spaces should be used to visually separate groups of numbers in international notation. In national notation, parentheses are used to indicate digits that are sometimes not dialled. Parentheses should not be used in the international notation. A slash (/) may be used to indicate alternate numbers.

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★ International Telecommunications Union Recommendation E.123