★ International Mobile Subscriber Identity - a unique identification that is associated with all GSM and UMTS network mobile phones.

★ The SIM inside the phone contains it as a 64 bit field.

★ Used for acquiring certain details in Home Location Register or Visitor Location Register.

★ IMSI is rarely sent for preventing eavesdroppers to identify and track the subscriber on the radio interface.

★ It is presented as 15 digit long number, where first 3 digits represent the Mobile Country Code and followed by the Mobile Network Code.

★ Remaining digits are mobile station identification number within the customer base network.