An international mobile subscriber identity (IMSI) is a unique number associated with GSM (Global System for Mobile Communications) and UMTS (Universal Mobile Telecommunications System) network mobile phone users used for GSM subscriber identification. The IMSI is usually 15 numbers and has two parts. In North America, the first part is comprised of six numbers and in Europe it is 5. These first numbers identify the country specific GSM network operator with whom the subscriber holds an account. The second part is given by the network operator to uniquely identify the subscriber. The IMSI is stored in the Subscriber Identity Module (SIM) inside the phone (aka SIM card) and is sent by the phone to the appropriate network. The IMSI is used to get the details of the mobile in the Home Location Register (HLR) or the Visitor Location Register (VLR).