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E.164 is an international standard ... E.164 defines a general format for international telephone numbers. ... In the United States, ...
There are many recommendations for recording phone numbers. The most common in the world is Recommendation E.123 E.123 International Telecommunication Union [11] and the standard format for Microsoft telephone numbers Microsoft. [12]
Each three-digit area code has a capacity of 7,919,900 telephone numbers (7,918,900 in the United States). Despite the widespread use of fictional telephone numbers of the form NYX 555-XXXX, only the block of line numbers from 0100 through 0199 are reserved specifically for this purpose, leaving the rest available for assignment.
Worldwide distribution of country calling codes. Regions are coloured by first digit. Country calling codes, country dial-in codes, international subscriber dialing (ISD) codes, or most commonly, telephone country codes are telephone number prefixes for reaching telephone subscribers in foreign countries or areas via international telecommunication networks.
Microsoft canonical address format for telephone numbers [2] [3] derives from E.123 international notation by allowing explicit indication of area code with parentheses. The canonical format is used by the Telephony API (TAPI) , a Windows programming interface for dial-up fax, modem, and telephone equipment.
In several European countries, a different strategy prevailed, known as the open numbering plan, which features a variance in the length of the area code, the local number, or both. [3] United States telephone numbers often included letter prefixes and telephone exchange names, which were more easily memorable for users than long digit sequences.
In the international telephone network, the format of telephone numbers is standardized by ITU-T recommendation E.164. This code specifies that the entire number should be 15 digits or shorter, and begin with an international calling prefix and a country prefix.
The implementation and expansion of the North American Numbering Plan between 1947 and 1992 preserved a long-standing practice in the United States and Canada that callers should only need to dial the local seven-digit telephone number when placing a call within the caller's exchange area or within the home numbering plan area (NPA).