An International Mobile Subscriber Identity (IMSI) is a unique number associated with all GSM and Universal Mobile Telecommunications System (UMTS) network mobile phone users. It is stored in the Subscriber Identity Module (SIM) inside the phone and is sent by the phone to the network. It is also used to acquire other details of the mobile number in the Home Location Register (HLR) or as locally copied in the Visitor Location Register (VLR). In order to avoid the subscriber being identified and tracked by eavesdroppers on the radio interface, the IMSI is rarely sent and a randomly generated Temporary Mobile Subscriber Identity (TMSI) is sent instead. An IMSI consists of three components:
Mobile Country Code (MCC)
Mobile Network Code (MNC)
Mobile Subscriber Identity Number (MSIN)
The traditional SIM card issued by a mobile operator contains a single IMSI for the issuing operator. The IMSI is then used to authorize access to mobile networks, whether the home network or another network the device has roamed.