got a call from this number tried calling it back and it says number is on the do not call list

This phone number is from a debt collection company located in down  town phx az .. The call is suspicious in that no one picks up which leads me to suspect a scam .  This needs to be addressed as many phone calls are being made from this same phone number and also its being listed as unknown in some cases.. ..  Hint if you call the number no name or even robo call identity ...