I had the same question, except mine was not alerting me on only a certain phone number. I tried everything mentioned in this thread. Nothing worked! I finally went to my contact info on the non-alerting number and pressed "edit". Toward the bottom of the screen when scrolled down there was a "do not disturb" button that was toggled "on". I Untoggled it and of course now I get the alerts! I had never set that up on a particular number - I wouldn't have known how! But now, when I go back into that contact and open edit, there is no "do not disturb" toggle switch! It's gone!
I was so frustrated, I just thought if anyone else had this problem it might save you from the same frustration! 🙂