I have a 2009 328i xDrive and my DINC loses connectivity all the time. No one's ever complained about the quality of my voice, but I notice that every few minutes the connection is gone, then all of a sudden it will pop up again. And sometimes when I turn the car on with bluetooth already on, the phone will not automatically connect.. I've never lost a connection while on the phone, but it's annoying having to set it up myself all the time.
I've read that the DINC's bluetooth profile is out of date and might not be compatible with certain bluetooth systems, but i find that hard to believe since my old iPhone 3G and Blackberry Bold 9000 had none of these problems with my BMW.
No clue on how to fix it (without rooting at least)