Try these 3 easy steps:
1) Go to: Settings > General > Accessibility > Incoming Calls
2) Change option from 'Default' to 'Headset'
3) Un-pair and re-pair the phone in the vehicle (or, reset and erase all previous pairings)
That's it. This worked for me when pairing 2 seperate iPhone5's in a 2013 CRV to the Bluetooth hands-free feature. I assume this would work for other Bluetooth pairing situations. Good luck and pass this on if it works for you.