I am struggling with a similar issue with my Acura RL, as you know. I had my car checked out yesterday at Acura. They did a diagnostic and said that there's nothing wrong with the car. I tried my 8700 in another Acura RL and it worked fine. My 8700 works fine in other cars and with a headset. So there is only one thing left to try. Exchange my 8700 for another and see if it works. (There is no logic in this. It's just that it's the only alternative not yet tried.)
I went to Cingular and they said if I exchange it, I have to keep it. My 30 day trial period will end. I said but it's defective - why should I lose my right to cancel within 30 days? They said too bad, that's the way it is. I asked them if I could try another 8700 in my car which was outside the store. They said no. Also they said they didn't think it was defective as it works with a headset as they demonstrated to me.