IMSI is an acronym for International Mobile Subscriber Identity. This is a unique number that is associated with all GSM network mobile phone users.
An IMSI is fifteen digits long, the first three digits are the country code (MCC), the next two are the network code (MNC) and the remaining ten digits are the unique subscriber number within the network's customer base.
Example
3598881234567890
359 - Bulgaria 888 - Mobiltel 1234567890 - Subscriber ID