I have a 2-month old Model S, had the same issues with my iPhone X, and couldn't get any satisfactory answers from Tesla. They kept telling me it was a firmware issue or iOS issue. Tried everything that was suggested (pairing & unpairing, rebooting my phone every time I stepped into the car, rebooting the MCU2, upgrading to newer firmware, etc.) but nothing worked and calls kept jumping between the phone and the car.
Separately, my 12V battery for my MCU2 died a couple of weeks ago and had to be replaced. As soon as I had a new 12V battery my problems with calls switching from the car to my handheld stopped and my Bluetooth connections held steady. I still have the issue that my contacts won't synch, but feels like a small victory that I can at least carry a conversation without the call jumping back and forth between the car and my phone.
So maybe ask Tesla if a weak 12V battery has anything to do with it?