How do I increase volume other than using side button on iPhone?

To change the ring volume on an iPhone you can go to Settings > Sounds & Haptics > Ringtone & Alert Volume and use the slider there.

Also, there is a "Change with Buttons" option immediately below that slider where you can turn off or on the ability to change the ring volume by using the side buttons.

Can’t hear phone ring

This is broken AGAIN on my iPhone 11, running 16.4.1a. The slider on the iPhone screen has no effect. I had to enable changing the volume with the side buttons in order to turn the ringtone up to its loudest setting.