Hi cliff111,
We understand you are experiencing an issue with unexpected callers attempting to join FaceTime calls.
It is possible that the phone number or email address being used for FaceTime is being sent fraudulent calls.
We would recommend setting Communication Limits to 'Contacts Only' under: Settings > Screen Time > Communication Limits > Allowed Communication > During Screen Time.
This will automatically block any caller(s) that are not in the Contacts List.
Alternatively, it may be worth looking into changing the phone number / email address used on the device.
Hope this helps!
Thank you for using Apple Support Communities.
Cheers!