The previous answer does fix Phone Ringtone "Settings" app When
1. "Do Not Disturb" is on?
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.