I have two iPhone 6 phones. On my 2015 Audi all road one works fine the other does not. They both work fine in my 2009 Audi A4.The symptom was that the phone would pair but no contacts were transferred.
Did not get much talking with Apple support -we did all the usual deleting and pairing again.
I spent a long time on this and the following worked for the iPhone that was uncooperative before.
This assumes the phone has been paired to the car via bluetooth.
1. Set the auto lock time on the iPhone 6 from the General option (I tried 30 secs and 5 minutes or never). The time selected is NOT important whatever you are comfortable with.
2. Have the iPhone display the desktop.
3. There are two options now. a) You use the phone in the car with the desktop showing or b) The auto lock shuts off the screen (according to your selection in #1 above). In either case the phone will work perfectly in the car. The auto lock functionality is important because it establishes that the phone will still be working as you drive. (Apparently when you proceed from the desktop the phone works fine via bluetooth in the car).
4. The phone will now work fine with bluetooth in the car and the contacts will be accessible.
5. If you now try to access the screen (lost the phone) when it has been auto locked you get the screen prompt asking for the password. This is perfect because the phone is secure. I should point out that if you select the "never" option for the auto lock then a lost phone can be used without your permission!