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.