You can't! You have to have a call blocker that you can buy online from Amazon or get one that is a computer program if you leave your computer on 24/7.
We got the Call Blocker and it blocks all "Unknown, Out of Area, and wildcards".
It also blocks up to 80 numbers.
We were getting Unknow and Out of Area calls all the time, and now it don't happen!
So forget Comcast for any help.