The MSISDN number is the telephone number that is assigned to a mobile user. This telephone number makes it possible for a caller on a wired telephone network to call a mobile station. The MSISDN together with IMSI are two important numbers used for identifying a mobile subscriber. The IMSI is stored in the SIM, and the MSISDN is the number used for routing calls to the subscriber. The SIM has a unique IMSI which doesn't change while different MSISDNs can be associated with the SIM.
There are slight variations in the MSISDN number depending upon whether one is from North America or other parts of the world. In North America, the number contains the CC (Country Code (+1)), NPA (Number Planning Area (Area Code)), and the 7 digit phone number. In the rest of the world, the MSISDN contains the CC, NDC (National Destination Code) which is similar to the NPA but can also identify the type of network (fixed, wireless, etc.) is being called.