It is the mapping of the telephone number to the subscriber identity module in a mobile or cellular phone.
This abbreviation has several interpretations, the most common one being "Mobile Station International Subscriber Directory Number".
The IMSI is often used as a key in the home location register ("subscriber database") and the MSISDN is the number normally dialed to connect a call to the mobile phone.
The MSISDN follows the numbering plan defined in the International Telecommunication Standard Sector recommendation E.164.
Minimum length of the MSISDN is not specified by ITU-T but is instead specified in the national numbering plans by the telecommunications regulator in each country.