have you got highline and/or the wiper stalk? if so you should be able to access the volume menu using the wiper stalk
1) make a bluetooth connection with your fiscon
2) make a call to someone
3) whilst its ringing - turn the volume knob on your RNS - it should increase the 'telephone' volume - thats one of the volumes
the other 'volume' option is of course the one inside the fiscon which would be changed using either the wiper stalk and/or MFSW buttons - this menu would only appear after you've made a bluetooth connection to your phone -
1) make a bluetooth connection with your fiscon
2) go to 'setup' in the 'fiscon bluetooth' screen on your MFA/FIS
3) find the bluetooth volume option and have a good play around!
good luck