I used the Motorola HF850 BT kit in my car (prior to upgrading to the OEM BT), and I thought it worked great!. Roughly$100 shipped from eBay, and mutes radio/CD when placing/receiving calls, decent voice dial features (if your phone has voice dialing) from your phonebook or just say numbers.
Here is a post on my install in the A6:
http://forums.audiworld.com/telematics/msgs/1153.phtml
I now have the OEM kit, which is fully integrated with the steering wheel controls and the RNS-E navigation, but it was a LOT more $$$. If all you need is basic handsfree, get the Moto or Parrot kits.
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2002 A6 4.2 Silver/Ebony, Tip, Premium, LOTS of mods!