The IMSI and MSISDN are two sets of numbers that uniquely identify a mobile phone. These numbers allow global access to the device through wireless technology. They are stored in the mobile phone's SIM (Subscriber Identity Module) card, which facilitates all activities connected to the mobile operator or carrier.
The MSISDN, or Mobile Station Integrated Services Digital Network, is a number that uniquely identifies a SIM card in a GSM (Global System for Mobile Communications) or a UMTS (Universal Mobile Telecommunications System) network. In other words, it is the number of the SIM card assigned by the mobile network. If you are calling a friend, for instance, the number you dial is the MSISDN.
IMSI stands for International Mobile Subscriber Identity. It is usually composed of 15 digits, with the exception of mobile phones based in South Africa. Those consist of 14 digits. The IMSI contains a Mobile Country Code, or MCC, Mobile Network Code or MNC and a Mobile Subscriber Identity Number or MSIN. This number is "burned" into the SIM card and serves as its "identity."
Difference Between MSISDN and IMSI
While they both pertain to mobile phones, the MSISDN and IMSI number are actually very different. The MSISDN is simply the phone number and the IMSI is the SIM card. A network cannot change the IMSI of a SIM card, but it can assign a different MSISDN, or phone number, to it.
The MSISDN and IMSI are used by network operators/providers to manage their network subscribers. With the use of MSISDN and IMSI, subscribers can change phone numbers without having to change the SIM card, or change SIM cards and still keep the same phone number and account information.