Definition
MSISDN
The full international mobile number that maps to a SIM — effectively the dialable number in E.164 form.
MSISDN (Mobile Station International Subscriber Directory Number) is the number people actually dial to reach a mobile subscriber. It's distinct from the IMSI, which identifies the SIM internally on the network.
In practice, the MSISDN is the phone number you submit to a lookup, expressed in international E.164 format.