An ITU-T (International Telecommunication Union - Telecommunication Standardization Sector)) standard that defines global telephone number format. As described in by the ITU, the E.164 general format must contain only digits split as follows:
Country code (max 3 digits)
Subscriber number (max 12 digits)
We often refer to a telephone number as a MSISDN – Mobile Station International Subscriber Directory Number – for example, a telephone number using international E.164 format. This term is used in quite a few messaging documents. It is sometimes pronounced as “miz-din.” The term: MDN – or Mobile Directory Number – also refers to a telephone number, using the international E.164 format.