I am having the same problem. My iphone drops calls. Same symptom all the time when it happens. Person calls, I answer, 2 seconds later, it drops. I replaced my first iphone xr. The replacement has the same problem. Any future replacements will be with a different phone or Verizon itself.
I might have to restart my phone many times a day as a pre-emptive measure until this is resolved.
I am really unhappy. I had three iphones before: the 4, 5s, and 6 plus. None of them had the drop call problem this one does.