BE AWARE: If you pick up on calls that are these marketing scams then you let them know that they have reached a valid phone number. They sell the numbers to other marketing scams. I now screen ALL calls. as I see it, if the call is important, then a message will be left.
I then save the number with a ZZZ-NoAnswer name, then next time they call, I will know by the look up and the ZZZ places them at the bottom of my phone list. Right now I have 64 of them.
As soon as it rings, they know they have a valid phone number. Otherwise they would get the pre-recorded message that they have reaced a non-working number. As soon as your answering machine picks up to screen the call, they know they have a residential number.