Answer: An international mobile subscriber identity (IMSI) is a unique number, usually fifteen digits, associated with Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) network mobile phone users. The IMSI is a unique number identifying a GSM subscriber.
This number has two parts. The initial part is comprised of six digits in the North American standard and five digits in the European standard. It identifies the GSM network operator in a specific country with whom the subscriber holds an account. The second part is allocated by the network operator to uniquely identify the subscriber.
The IMSI is stored in the Subscriber Identity Module (SIM) inside the phone and is sent by the phone to the appropriate network. The IMSI is used to acquire the details of the mobile in the Home Location Register (HLR) or the Visitor Location Register (VLR).