E.123

E.123 is a 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.

In the international telephone number notation, the leading plus (+) serves as an international prefix symbol, and is immediately followed by the country code. The user or the telephone system should replace the + symbol with international dialing prefix used in the caller's location.

Parentheses are used in national notation to indicate digits that are sometimes not dialed, such as area code in variable-length dialing numbering plans. Parentheses are not allowed in the international notation, according to the standard, as international callers use fixed number dialing.

For digit grouping, E.123 specifically recommends that:

Digit grouping rules should follow established national conventions.

In national notation, the trunk prefix can be included with the area code if required by national writing conventions; trunk prefix is included in most European countries whenever they use fixed or variable dialing, but is omitted in USA and Canada where phone numbers only indicate optional area code.

A tilde (~) indicates an additional dial tone that the user should wait for.

A slash (/) with spaces on either side may be used to indicate alternative ending for numbers (i.e. "555 1234 / 4444" means 555 1234 and 555 4444.)

The non-dialable PBX extension number should be separated by words "extension" or "ext." in the national language after the phone number.

When the PBX is capable of direct inward dialing, the extension number should be written directly after the phone number, without using any distinct symbols. If there is a need to indicate in-dialing capability of the telephone number, a number of dots (....) corresponding to the length of the extension number can be added at the end.

Microsoft canonical address format for telephone numbers derives from E.123 international notation by allowing explicit indication of area code with parentheses.

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