iPhone not ringing

It's the little physical switch on the upper left side.

Another common "cause" of non-ringing is that the phone has been inadvertently placed in "Do Not Disturb" mode.

You can read about that feature here: https://support.apple.com/en-us/HT204321

Doh:

On re-read ... I see you asked about "unMUTE"

The "Mute" "button" appears on the iPhone's dialing screen once a call is in-progress.