A couple of times I have discovered that my ring volume was off/at zero, and that I had missed calls.
When I held volume up, a volume slider appeared but didn’t affect ring volume.
(apparently only changing media volume)
I had to raise media volume, then quickly tap the setting icon at the bottom of the volume slider. This brought up several types of volume levels, and I was able to change ring volume (which was at zero and not raising via volume button)
I am not sure that I didn’t accidentally lower the volume to zero, but it should be easy to raise it back – not requiring one to open volume settings.