IMSI number. In gsm network, the only number to identify a mobile station is the IMSI number, that is, the international mobile subscriber identity. The IMSI number is a digital identifier of 15, which is used to uniquely identify the mobile subscriber in the mobile network. Each IMSI number is associated with a SIM card, which is inserted into the mobile device to enable the device to connect to the GSM network and communicate. In addition to the IMSI number, the mobile device also has a unique identification code IMEI number, which is used to uniquely identify the device itself rather than the user associated with the device.