International mobile subscriber identity

The International Mobile Subscriber Identity or IMSI /ˈɪmziː/ is used to identify the user of a cellular network and is a unique identification associated with all cellular networks. It is stored as a 64 bit field and is sent by the phone to the network. It is also used for acquiring other details of the mobile in the home location register (HLR) or as locally copied in the visitor location register. To prevent eavesdroppers identifying and tracking the subscriber on the radio interface, the IMSI is sent as rarely as possible and a randomly generated TMSI is sent instead.

The IMSI is used in any mobile network that interconnects with other networks. For GSM, UMTS and LTE network, this number is provisioned in the SIM card and for CDMA2000 in the phone directly or in the R-UIM card (the CDMA2000 analogue to a SIM card for GSM).

An IMSI is usually presented as a 15 digit number, but can be shorter (not longer). For example, MTN South Africa's old IMSIs that are still being used in the market are shown as 14 digits. The first 3 digits are the mobile country code (MCC), which are followed by the mobile network code (MNC), either 2 digits (European standard) or 3 digits (North American standard). The length of the MNC depends on the value of the MCC, and it is recommended that the length is uniform within a MCC area. The remaining digits are the mobile subscription identification number (MSIN) within the network's customer base (mostly 10 or 9 digits depending on the MNC length).

...
Wikipedia