International Mobile Subscriber Identity (IMSI) Number

The IMSI is a unique non-dialable number allocated to each mobile subscriber. This is a unique number that is associated with all GSM and UMTS network mobile phone users.

The IMSI is made up of three parts
(1) the mobile country code (MCC) consisting of three digits,
(2) the Mobile Network Code (MNC) consisting of two digits, and
(3) the Mobile Subscriber Identity Number (MSIN) with up to 10 digits.

IMSI is a unique number that is associated with all GSM and Universal Mobile Telecommunications System (UMTS) network mobile phone users. The number is stored in the Subscriber Identity Module (SIM). It is sent by the mobile phone to the network and is also used to acquire other details of the mobile in the Home Location Register (HLR) or as locally copied in the Visitor Location Register. In order to avoid the subscriber being identified and tracked by eavesdroppers on the radio interface, the IMSI is sent as rarely as possible and a randomly generated TMSI is sent instead.
see here