The previous answer does fix Phone Ringtone "Settings" app When

1.  "Do Not Disturb" is on?

1. If you go to "Settings", and from there, go to "Sound", you should see a "Do not Disturb" tab in the "Notification" section. Click on that, and you'll see an on and off switch. 

2.  I experienced a "New Condition" where phone ringer turns off; 

When Phone setting in sound control has  "Vibrate"  ON- automatically triggers (in error) the "Do not Disturb" tab in the "Notification" section.

Could be a software or hardware problem ** My fix is to never allow "Vibrate" turned On.

 

Mine was on ‘vibrate when ringing’ in error. Disabled and all fine.

Did you check to see if "Do not Disturb" was on? If you go to "Settings", and from there, go to "Sound", you should see a "Do not Disturb" tab in the "Notification" section. Click on that, and you'll see an on and off switch. Hope that helped!

Why won't my Galaxy Prime phone ring when someone is calling me?