National conventions for writing telephone numbers

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National conventions for writing telephone numbers vary by country. The International Telecommunication Union (ITU) publishes a recommendation entitled Notation for national and international telephone numbers, e-mail addresses and Web addresses. Recommendation E.123 specifies the format of telephone numbers assigned to telephones and similar communication endpoints in national telephone numbering plans. In examples, a numeric digit is used only if the digit is the same in every number, and letters to illustrate groups. X is used as a wildcard character to represent any digit in lists of numbers.

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