Did you try turn off the voice privacy feature?
This solution this Dude was trying to convey by saying there is an issue by turning up the Volume on a Call my Droid Maxx works that way my Dad's work phone which is a LG flip works that way as well.. you turn the Volume up while off a call all your doing is Turning up the Ringer Volume..
What a Goof.!
I experienced this right after purchase, turned volume all the way up but no joy. Then the solution, you must turn the volume up while on a call. Did it and no more problem. Too obvious but just try it.