I have the same problem but have found a way to improve it slightly.
The problem occurs when there is too much noise at either end. It is particularly bad if you are on a conference call with multiple speakers at the other end. It's also bad when I'm talking to my wife who's in the kitchen with the washer on. I just think the incoming sensitivity is too high. The only way to fix that bit is to ask the person to go somewhere quieter.
It also happens if I'm on a 1-2-1 call & I have the volume inside the car up high. The Zafira's a re noisey beasts so it is difficult to hear. If you turn it up[, it causes too much feedback & the car kit cuts it out.
Sorry I can't help with a better solution, I just hope Nokia take what I've said into consideration.
I have been using my 6021 without the cradle (just bluetooth sync'd). I'm tempted to buy a cradle to see if that makes it better. However, at £90 for a bit of plastic, it doesn't seem worth it!