How to increase volume on incoming calls?

Go to settings > change with buttons, & make sure it is toggled ON. Then go to your home screen & while all apps are closed, (doesn’t matter if they keep running in the background) press & hold the ‘volume up’ button. You should see the ringer volume going up at the top pf your screen. Also, make sure you have the iPhone removed from ‘silent mode’. To put your iPhone in Ring mode, move the switch on the side of your iPhone so that orange isn't showing. One more thing, if you use assissive touch on your iPhone, you should make sure you haven’t silenced your iPhone from there.

You can read further about ringer & silent modes here:

How to put your iPhone on vibrate, ring, or silent mode - Apple Support

What happens when you try the volume up button?

I’m glad it worked out for you.