How do I turn Ringer Volumne Up?

When a call comes in on my iPhone the ringer volume displays momentarily. It's at the lowest setting. If hit the side button on the phone to raise the volume, the ringer volume indicator is dismissed and is replaced by the system volume indicator. I cannot find a way to raise the ringer volume (no, it is not associated with the system volume).

One would think this would be simple, but I cannot find the ringer volume control anywhere.