First of all, what is IMSI? IMSI stands for International Mobile Subscriber Identity. If IMEI number refers to the unique identity of the phone, IMSI actually represents the unique identity for the subscriber of the phone including the origin country and mobile network that the subscriber subscribes. It basically identifies the user of a cellular network and every cellular network has its own unique identification.
In layman’s term, if IMEI can be considered similar as the plate number of your car, IMSI represents the unique identity of whoever is driving the car. Basically, all GSM and UTMS networks use IMSI as the primary identity of a subscriber or user. The number that represents IMSI can be as long as 15 digits or shorter. The first three digits are the mobile country code and followed by the mobile network code. The information of IMSI is also contained in the SIM card. IMSI are normally used by network operator to examine the subscribers and whether to allow the subscriber to use another network operator.
Law enforcement has been using certain technology such as Stingray or Triggerfish to track the IMSI number of a subscriber. By using the technology to track your IMSI, the authority can actually track not just the location of your phone but also who you are calling, at what time and where the call is made. The Stingray will act as a fake cell towers that then will trigger your cell phone to communicate with it. Once communication is established, the technology will able to even capture your conversation. This technology is controversial however as innocent public probably do not want their conversation to be captured or heard. Triggerfish technology envisions a scenario that usually happens in an espionage movie.