I had the exact same problem. I called AT&T and it was fixed immediately. I don't remember exactly what we did, but it was a very minor issue to resolve.
I did that. Worked the first time, but went back to not ringing after that.
DND is off as well. All other notifications are working except incoming calls. The weird thing is that sometimes the volume up/down toggle is sometimes stuck at silent and I would have to restart the phone to be able to adjust the notifications volume.