iPhone 15 iOS 17.2.1 ringer volume

If your phone is in a case, check to make sure it the same issue occurs when the phone isn't in a case.

If your phone is not in a case, make an appointment to have your device examined, as it may be a hardware failure:

Genius Bar Reservation and Apple Support Options - Apple

Alternatively, contact Apple Support directly:

Contact - Official Apple Support

Ive lost the ability to use the increase volume button at all….decrease volume works fine but increase volume has stopped working, so I cant increase volume while in apps/or to adjust notification sounds and can no longer do screenshots.

I have the same issue, and it's quite annoying. I switched from Android recently, and tomorrow I'm going to return the phone to the store.