My phone has no volume on incoming calls. I am missing all calls, many important. I wasted 3 hours at the Apple store only to be told it is my carrier. I don't believe so now as, it looks like you are aware of this issue. Have you found a fix yet?

Do the missed calls appear on your Recent Calls list? If not, they aren't getting to your phone at all, and you need to work with your carrier to resolve the problem.

If the missed calls DO appear in your Recent Calls list, look in the status bar. Do you see a crescent moon, meaning Do Not Disturb is enabled? If so, go to Settings/Do Not Disturb and turn it off, or change the schedule.

Also check the Mute switch on the left side of the phone, and make sure the ring volume is up. You adjust the ring volume with the Home screen displayed, using the volume buttons on the left side.