Is this during WiFi Calling only?
If this happens with WiFi Calling only, try getting a personal cellspot from T-Mobile (or buy an ASUS router)
Google App takes over your microphone if the voice detection is set to "Always On". Normally it should release the microphone to calls but older versions of the App did have a bug so the mic got jailed by the Google App.
Google App (OK Google) > Settings > Voice > "OK Google" detection
Turn On "From the Google App"
Turn Off "Always On"
Wifi calling is not being used just regular calling, I'll try these suggestions and get back to you too. The phone battery is also not charging over 50% with stock charger.