Same issue here with the Hyundai i20 Bluetooth system.
Display shows phone connected via Bluetooth, speakers work fine, microphone works fine, Siri is heard over the speakers and can be used to voice activate outgoing calls plus other Siri commands.
Music can be played via Bluetooth aswell, appears as MP3 play, so the connection is live and the display reflects the media source.
If I switch Bluetooth off on the iphone then the display says no connection, if I try to dial a number.
So again the connection to the vehicle isn't a problem.
However once connected if I press the Green dial or voice activated commands on the main console of the car, nothing happens if somebody calls then I can't answer as the console commands are inactive.
If I use Siri to call a number, which does work for outgoing, then the active callre display shows the last number the car received, which is incorrect as the active call is initiated via the iPhon, regardless of the number in use.
Please note other manufacturers phones work normally when connected to the Bluetooth system so all indication is it's the iOS8 update which has caused the issue as iOS 7 worked perfectl.