Usually nothing....I just wake. Sometimes an outside noise will do it. But I don't hear the telephone or doorbell when I'm sleeping. If someone turns on the TV that will wake me in a fury.
Living in NYC, there are endless things that could be the reason I wake up before my alarm goes off - sanitation truck at 2:30 AM, big delivery trucks, cars with these new-fangled twin exhausts that sound like gunshots, pigeons cooing for their mates and more.