Hello wassnert,
To block an unwanted caller on iPhone please review the following user guide: Block unwanted callers in FaceTime on iPhone - Apple Support
"Block unwanted callers in FaceTime on iPhone
In the FaceTime app, you can block FaceTime calls from unwanted callers.
- In your FaceTime call history, tap
next to the name, phone number, or email address of the contact you want to block. - Scroll down, tap Block this Caller, then tap Block Contact."
If you're unable to decline the call at this time you may be able to restart your device to continue with the steps above.
Regards.