The fraud alert feature lets users report fraudulent attempts to access their resources. When an unknown and suspicious MFA prompt is received, users can report the fraud attempt using the Microsoft Authenticator app or through their phone.
The following fraud alert configuration options are available:
Automatically block users who report fraud.
Code to report fraud during initial greeting.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/authentication/howto-mfa-mfasettings