E.123 is standards-based recommendation by the International Telecommunications Union sector ITU-T, and is entitled Notation for national and international telephone numbers, e-mail addresses and Web addresses. It provides guidelines for the presentation of telephone numbers, email addresses, and web addresses in print, on letterheads, and similar purposes.
Example formats 
|Telephone number, national notation||(042) 123 4567|
|Telephone number, international notation||+31 42 123 4567|
|Web address / URL||www.example.com|
E.123 specifically 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 (/) with spaces on either side may be used to indicate alternative numbers. (i.e. "555 1234 / 4444" means 555 1234 and 555 4444.)
Emergency contact information 
A standardized language-independent way to identify a next-of-kin (or other emergency contact) in a mobile handset’s directory, in case of an emergency, has in May 2008 been adopted as a new clause in Recommendation E.123.
It proposes to store emergency contact numbers prefixed with Arabic numerals in the form “0nxxxx”; “n” is a digit from 1 through 9 and “xxxx” is any meaningful descriptive character string in any language or script (e.g. “Anna” or “Spouse”).
In the handset's directory this would be displayed as "01Anna" or "01Spouse" enabling easy identification by the emergency services. The handset’s directory entry (in the “contact number” field) would contain the actual number of the person to call in case of emergency.
See also 
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