I see a lot of phonebook problems on this forum. I've got an answer here for a specific problem.
I have an A6 2006 with MMI and "phone prep". Cell phone is LG VX8600 (CDMA), it's on the compatible list.

Bottom line: the car WON'T download or synch with the phonebook if you've got your security lock code enabled. This is true even if you have previously entered your lock code. You MUST disable the lock code functionality.

In my case, the original phonebooksynch worked fine. Then I enable the lock code on my phone. I later noticed that my phone directory as seen on the Audi MMI wasn't updating; It was showing the original download too. At first, I thought I had too many entries, so I reduced it below 150. No dice: neither the new entries added or the old entries removed from the MMI. I cleared all the Audi memory, nada. I even downloaded new code to phone. nada. I thought maybe the powerup sequence mattered, nada. I tried disconnecting and re-pairing, nada. In all cases, I entered the lock code on the phone (set to powerup) as soon as it powered up. nada.

As soon as I went into the phone settings and disabled the lock code, I saw, within seconds, all my phonebook entries.

I can't say this will help everyone, but it might give you a clue where to look for settings.

What really drives me crazy about both the Audi and the phone manufacturers is how damn secretive they are in describing exactly what bluetooth features are operational. Broad-sweeping statements like "Bluetooth 1.1", or "phone support" are not helpful. It makes interoperability nothing but a big guessing game. Audi should pay a contractor for a 2-week research project to take every phone in production and try every feature, then build the mother of all compatibililty charts. Repeat annually.I'm not sayingfix what doesn't work, justtell us what does. It would save us all a lot of time. It wouldn't cost that much and would give the luxury market, which Audi purports to play in, a distinct competitive advantage.

I hope this helps someone.
Good Luck.