I just don't see how, if the bluetooth is playing thru the headunit, and the DSP is only seeing signal from the headunit, the DSP would be muting the sound. The only thing I can figure out is that the music is stereo and maybe the phone call is mono. If both channels is playing a mono signal, you could be getting cancellation.
Here's an experiment, try muting one side and making a call to see if it's louder.