Volume control for Phone

Almost hate to post this but the volume when I answer the phone through my headset I can not turn up volume for phone conversation. Everything is up as loud as it goes, radio/XM/media all work and are loud and clear. Phone rings and I answer it and can hear the person but volume is set low that I can't understand what is being said? Read the book repaired and no luck. I have a Schuberth C-4 (built in) and running a IPHONE 11 ProMax. First issue on a BMW that the volume stump me.
 
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