call screening - How to block private numbers

I am getting way to many calls from marketers these days. I have listed my number on no call list and they still keep calling. Most of the time I can't be bothered to report everyone.

How to block callers from private numbers that are hiding their Id?

Answer

On Android version 5.1.1 (Samsung Galaxy S6 Edge)
One way to do it is go to Phone (like you would make a call) then click on the More in top right of the screen and choose Settings then Call Settings > Call rejection > Auto reject list and select the "Unknown" option.

Hope this saves you some time.