IMSI = International Mobile Subscriber Identity

IMSI is the identification number of a mobile phone subscriber which is unique around the world and cannot be duplicated. It is different from MSISDN, and is stored as a 64 bit field stored which is sent to the network by the phone. IMSI contains mainly 3 parts, MCC, MNC, and MSI. For example, the IMSI of a Grameenphone, an operator in Bangladesh will be 470017XXXXXXXX, where 470 is the MCC (Country Code), 01 is the MNC (Network Code), and 7XXXXXXXX is the MSI(Mobile Subscriber Identity). The first digit in MSI refers to operator in Bangladesh.

For more detail about IMSI, please check this link from wikipedia

https://en.wikipedia.org/wiki/International_mobile_subscriber_identity