Kenwood KDC-BT838U -- Low audio on incoming phone calls only.

Just installed Kenwood KDC-BT838U in Honda Civic with iPod cable and steering-column-mount microphone. BT registered OK & connected with cell phone; can now answer cell phone calls. The outgoing mic volume is basically normal (but sounds choppy to the party on the other end, as is typical with these units -- do you know of any way to improve it?); however I cannot hear incoming callers. The incoming call volume is EXTREMELY LOW over speakers -- I can hear just a faint murmur of the incoming caller's voice, which was basically inaudible. This problem *only* occurs with incoming phone calls; the volume of radio stations sounds fine over the speakers.

Thanks in advance for any advice you can offer!