Explanation

Code to report fraud during initial greeting: When users receive a phone call to perform two-step verification, they normally press # to confirm their sign-in. To report fraud, the user enters a code before pressing #. This code is 0 by default, but you can customize it.
Block user when fraud is reported: If a user reports fraud, their account is blocked for 90 days or until an administrator unblocks their account. An administrator can review sign-ins by using the sign-in report, and take appropriate action to prevent future fraud. An administrator can then unblock the user's account.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/authentication/howto-mfa-mfasettings#fraud-alert