iphone5 car audio problems

Hi - hoping someone here can help.

I have an iphone5 running iOS 6.1.4 and a 2013 BMW 3-series. I

can get both to pair/connect via Bluetooth, can pull-up my Contacts list from my phone on the BMW iDrive screen, and can even initiate a call. But what I can't appear to do is get the audio to jump from the phone to the car - even though the symbol on my iphone is showing the 'speaker' against the car.

When I try to make an outbound call from the car, there is no dial-tone or ring-tone. The only difference when I receive an incoming call, is that I hear it ringing, but there is no audio when I pick-up....and the caller cannot hear me speak either.

I have tried the exact same procedure with my iphone4 in the same care, and it works perfectly over the car audio/speaker - both for inbound and outbound calls.

If anyone has any clue as to what might br wrong, and how this can be fixed, I'd very-much appreciate it.

Thanks

Nick