ITU-T E Series Recommendations

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E.123 is an international standard by the Telecommunication Standardization Sector of the International Telecommunication 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.

E.161 is an ITU-T Recommendation that defines the arrangement of digits, letters, and symbols on telephone keypads and rotary dials. It also defines the recommended mapping between the basic Latin alphabet and digits (e.g., "DEF" on 3). Uses for this mapping include: