Not sure...what ringer/volume profile is your phone set to when you're in the car? I have an 8830 and using Ford Sync as well with absolutely no problems. Often people can't even tell I'm in the car talking through Sync.
Seems to me that if you have the same problem with two different phones, it's probably not the phone. I know the dealer said all is OK, but then again who knows. You should connect your 8830 to your friend's car who also has Sync - if the problem is there it's a bad phone...if not it's your car.