When I get a phone call I have a green answer box of red and a box and a clear screen call. When I push the green answer box and all three disappeared and it goes back to the home screen and no matter what I push I can't answer that call.
Does this happen with the phone locked or unlocked (or both)? Are you using any 3rd party phone apps (i.e. carrier spam filter options, 3rd party apps, etc.)? Does the same happen when placing a call or just receiving them? I would try to locate the Phone app on your phone, tap and holding it, go to the App settings and in the Storage section, clear cache. If that doesn't work, try clearing data. HOWEVER, make sure that you have any phone-related data backed up elsewhere (i.e. call history) as this will wipe that data off your phone as well as any settings you may have changed for call making/receiving. I would also make sure you've backed up contacts as well just in case (shouldn't be affected, but better safe than sorry).