E.123 is an international standard by the standardization union (ITU-T), 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. As described by the ITU, in E.123: "+" is the "International Prefix Symbol" used in a "Telephone number, E.123 international notation". Example formats Telephone number 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: 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; spaces should separate country code, area code and local number. No recommendation is made for grouping rules for digits in the local number, instead some examples of commonly used groupings are shown. 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. means and ). The non-dialable PBX (private branch exchange) 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