I have recently purchased a iPhone XS Max and have been having the same Face Time problems as you. When receiving a face time call I have the options to “Decline” or “End and Accept”. Both of which decline the face time call.
I called Apple for help, but they just referred me to my nearest Apple Store and could not offer any help or assistance
However, I figured out how to fix this problem and it’s simple.
Go to:
Settings then FaceTime
You will see that is says “Use your Apple ID for Face Time” or you will be signed in and it will show your Apple ID for Face Time calls.
To fix the problem, from this screen simply sign out of your Apple ID Account. Make it to where below that it says “you can be reached at face time anytime by” and it should have your cell phone number as the only option instead of having both your cell phone number and Apple ID. This will fix the issue. You should now be able to answer your face time calls.
Let me know if this works for you!