Some areas of application
The IMSI (international mobile subscriber identity) is a number that uniquely identifies a mobile network user, and it is stored on the SIM card. If the user changes a SIM card, his phone number (MSISDN) is retained, but the IMSI changes. Knowing about a change in IMSI allows you to keep track of any SIM card replacement.
How can this be used? To prevent possible fraud, which is especially important for financial institutions.
For example, before sending a one-time password, the recipient's SIM card is checked to see if it has been changed. If the SIM card has been changed, the message will not be sent. To resume receiving messages, the customer must be verified to ensure that the SIM card has not been taken over by fraudsters.