MSISDN is a Mobile Station International ISDN Number. MSISDN is a number uniquely identifying a subscription in a GSM or UMTS mobile network. It's a phone number of the SIM card.
The MSISDN together with IMSI are two important numbers used to identify a mobile phone. IMSI is often used as a key in the HLR ("subscriber database" or home location register) and MSISDN is the number normally dialed to connect a call to the mobile phone