The gravity has several different volumes that you can control, so that phone, music, nav, etc. are at different levels. In addition, for some reason the way they're implementing bluetooth they are allowing the device to control volume independently of the car, which is confusing. As you found with music, when on a call, grab your phone and turn the volume all the way up - on the phone - then make further adjustments using the steering wheel or volume wheel on the car. This allowed me to make it plenty loud to hear my calls, they just still couldn't hear me very well as other owners have reported. Either the Gravity has a crap microphone, or they need some more software refinement (fingers crossed) to improve it. I also only had to do this once, then my phone I believe remembers the last volume for each bluetooth device?