Pushing the plus/minus buttons on the exterior does not change the ringer volume when the cell phone is ringing. There is no menu option for ringer volume adjustment under the menu "settings / personalize / sound profiles".
Select "Menu / Settings / Personalize / Sound Profiles / Normal" , then select "Options", then select "Edit", then select "Ringtone Volume", then select "1- Change", then increase the indicated (bar graph) ringtone volume using the right-side Navigation key.