Might be old news by now, but
Hi
Had same problem-2 solutions
1-on the left side is a swith above the volume control-if" the orange is visible it is set to "SILENT"
2- going to settings-general-accessibillity-scroll to assistive touch and turn it on.
Then you have a white circle on black square icon -tap- tap on device- tap on Mute/Unmute- Try and see if that solved the ringer problem. If it has, go back and turn assistive touch off- that might also solve the problem.
Option 1 is the most likely cause for not ringing I discovered, but option 2 might reveal the hidden muted sound issiue as well.
Hope this helps