Random number calling on do not disturb

Hey, there's this one number that keeps calling me at 5:40am for some reason. I don't want to pick it up because i don't usually pick up random numbers. Whenever it called me I was on do not disturb mode and it worked. This morning though, it started ringing and it woke me up. I still was on the don't disturb mode and I checked if my settings were on right and they were. I can only have my favorite contacts to call me, and that random number is definitely not in my favorites. Anyone know what happened? Thank you.