I am having this problem also:
When attempting to answer a phone call, the screen says "Call Declined" and hangs up.
Outgoing calls work ok.
When I reset my network preferences on the phone, the call answers correctly one time, then fails every time after that. I tried updating to the newest iOS (13.2) after the problem began occurring, but this did not have any affect on the problem. Have only had the phone a week and it started this error a few days after getting the phone. I am thinking maybe the modem or an antenna may be to blame?
It almost looks like it is trying to answer and forward to voicemail at the same time. (see below)