Can understand your reluctance about letting the dealers get involved.
Try positioning your phone somewhere else in the car and see if that helps. I use a Brodit phone holder which simply 'clamps' around the A pillar so I can see the screen and it also means the phone can be charged. On the occasions when I did forget to switch the bluetooth on and got a call I simply swithched the phone's loudspeaker on and could carry on the conversation without it going through the car's system.
If re-siting your phone does not work I would suggest that before going the dealers you 'pair' another phone with the car and see if you get the same symptons. Which if you do, will help convince the dealer it is not a phone issue.