Hi Steve. We have also had users complain of this. Usually the fix is to go into the profile on the app, then Settings, then Incoming Calls, then Sound, then choose a different ring tone than the default ringtone. The ring tone will start ringing. Use the volume buttons on the side of the phone to increase the ringer volume to the desired level and hit OK.
We don't get cellular calls on our devices as well so I can't say if this also changes the ring volume of your non-8x8 calls, but this is how we have fixed it when users complain that they can't hear the ringer for the app.
Eric