An IMSI (International Mobile Subscriber Identity) is a globally unique number it is used to authenticate the device on a GSM or UMTS network. IMSI is the acronym for International Mobile Subscriber Identity: a globally unique number that cellular networks use to identify and authenticate every user and/or device on a GSM or UMTS network. IMSIs are used in any network that connects with another and are stored on the SIM card. A 16-digit IMSI consists of three elements: • Mobile Country Code (a three-number code for the home country) • Mobile Network Code (identifier of the home network operator) • Subscriber Number (unique identifier assigned by the mobile network operator).