That's because most of these people spoof their numbers and just use random numbers. Sometimes they'll even use legit business numbers. I had a number call me one time and when I called the number back, it was a business owner explaining to me that he has been getting calls all day and hasn't called one person.
Interesting. I recently registered car, and was asked for number. Was quoted the wrong number as that which was listed for me. I found it odd at the time, as the number given was never one I'd had. DMV in cahoots with the spammers now?
This number rang my cell today. I picked up the phone on the 2nd ring, just to look at the screen and ID who was calling. Then it stopped ringing. I never "answered" the call. When I looked back at my call history it showed the called me 2x's, one minute right behind the next. I only heard it ring once. Strange.