Thanks for reaching out to the Apple Support Communities.
I see that you have no sound in the car when you answer a call, but the car makes a call perfectly. It's important to make sure that you can make and receive calls without any issues in your car, so I'm happy to provide some help.
Did this just start happening? I suggest following the steps in this support article: Get help connecting your iPhone, iPad, or iPod touch with your car stereo , specifically:
6. Install any firmware updates for your car stereo.
Let me know if you have any questions.