Same problem. 315 nav/bluetooth unit.
Seems the voice command and telephone volume do not match up. If I have the volume set up high enough to hear the phone the voice prompts are WAY TOO LOUD. If I set the voice prompt to a comfortable level, I can't hear the other end of the phone.
The manuals are pretty lame at explaining many of the features and controls, often with incomplete sentences and typos. Ccbb does a way better job here, but it is simply impossible to cover everything.
Anybody else have this problem? iPhone 3GS.