Hi all,
I have exactly the same problem as Dantonic, was working perfectly one day, the next it pairs, allows the call to go through the hands free device, but no actual sound. Very strange. The hands free device is ok as i have tested it with anothwer phone. Something has happened to the Hands free bluetooth connection.
OK, so I filed a bug for this (link in my signature)
One of the nokia reps... suggested to check a few things including the call volume while making a call while paired.
Apparently somehow the systems had automatically set my volume to 0. I guess there is a separate volume when paired to BT.
So try connecting, initiating a call, and raising the volume on your N900.