A4 Cab 3.2 S-Line Quattro
I had the exactly the same problem with the same phone and thought there was an intermitant fault with the Bluetooth, so took the car back to Audi, who told me that the phone wasn't compatible.
The Audi dealer printed me a list of phones that are compatible, which i would be more than happy to scan in and email across if you can't get the list from your local dealer.
For what it's worth, i have a friend who works for O2 and she said that Audi blutooth car kits are an absolute nigtmare!
Hope this helps