IPhone 6+ bluetooth phone speaker not working in my cars

Question

I have a iPhone 6+ with iOS 8.0.2 that I am having problems with both of my cars hands-free bluetooth telephone connections.
On both cars, the phone pairs just fine, and plays streaming audio fine, but when I make a phone call, the speaker in the car does not play the telephone call audio.  The icon on the phone shows the the speaker connection as bluetooth, but no audio comes out through the cars speakers.  The display on the car also will display "call failed" even though the phone is ringing.
I also have an iPhone 5S with iOS 8.0.2 and it DOES NOT have this problem.
The cars that I have tested this on are a 2014 Nissan LEAF, and a 2010 VW GTI.  On enthusiast forums for both cars, there are reports of this happening to other iPhone 6 users as well.
Thoughts or solutions?

Answer 1

I just ran an experiment with my old iPhone 5, where I reset it to factory settings, and then tried to pair it with my VW, and I was successful in getting that to work.
Now the question is how can I completely flush the bluetooth cache in my iPhone6+ that I restored with the contents of my iPhone 5 WITHOUT having to start from a factory fresh copy of iOS 8.0.2 and manually repopulating the iPhone6+ with all of my apps, folders, and OS settings.
I have already tried under General>reset>reset all settings, and >reset network settings, and neither of those reset the bluetooth settings.