The International Mobile Subscriber Identity (IMSI) is an 18-20 digit number uniquely identifying each SIM card.  The information can be used to identify, track or clone a subscriber and is sent as rarely as possible.  Instead,  a randomly generated Temporary Mobile Subscriber Identity (TMSI) is used whenever possible.

The first 3 digits of the IMSI represent the Mobile Country Code followed by 2-3 digits for the Mobile Network Code.  The remaining digits are the Mobile Station Identification Number (MSIN) assigned by the network provider.