Internet works sometimes but can't make calls all the time

I had a “similar” issue with my iPhone where sometimes I could have a call in progress and the person I was talking with could hear me but I could no longer hear them. I figured it out but it was pretty simple actually. I had my phone so it would connect to my car, some headphones, and some earbuds and a bluetooth receiver on a Bose speaker tower. What I found was the Logitech bluetooth receiver unit for the Bose speaker would hijack my sound from my AirPods or anything else. The other catch was if my daughter started my car to use it that would also possibly hijack my audio for a call. It can be very transparent and happen that your audio might be getting hijacked by another device. It seemed like the call was lost but only if I looked at my phone did it register that the call was up but audio was transferred. Only the speaker, not the microphone. 

Check and make sure your audio is not jumping to another device adding confusion. Just a thought.