I have not been able to set a default ringtone for the longest time. I tried everything but a factory reset which I did not want to do at all. I just found the easiest way to fix the problem. I have contacts group u, which is an app to group contacts (obviously) after I kept trying to set a default in my ringtone apps and fix it in settings, I selected all my contacts in the contact group app and set the ringtone. It was a hell of a lot easier than selecting each contact one by one and now my phone actually rings. I hope this solves the problem for everyone else, it worked for me. If you have any questions feel free to ask.