How are you answering your phone in the car? Are you sliding the "Side to Answer" button on the phone itself or are you using the steering wheel button.control to answer the phone?
I found when I switched from my Android to an iPhone that I had to now answer the phone, when connected by Bluetooth in my car, by pressing the steering wheel button/control (or additionally for me because I have a GM vehicle I can also press the OnStar button on my rearview mirror). When I tried sliding the button on the phone I would always disconnect the calling party. I had to re-learn how to answer my cell phone in the car.