Difficult to hearing phone calls

You cannot readily see the level if you increase it while making a call.

Call someone, and while they talk press the volume up button repeatedly til it is comfortable. Listen and talk volume cannot be adjusted at any other time.

click here: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker

or How to arrange  a repair

First try a Forced restart

https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

If no better

You should arrange a service or repair

https://support.apple.com/iphone/repair/service

Anything other than Screen or Battery may be a replacement phone and the price is in the article.

If not covered by warranty and you don't have AppleCare+, you may have to pay full replacement cost for severe damage 

You can see additional service information within the following article: 

Get your iPhone, iPad, or iPod touch ready for service - Apple Support

We’re volume settings adjusted to max DURING a live call?