We use the DIgitone Call Blocker that you can get on Amazon. Right out of the box it will block all "Unknown, Out of Area, and No Number" calls without doing anything. Then you can block up to 80 numbers. Also it will block out of country numbers, all the calls that Comcast won't block!
There are other blockers including one that you download on your computer, but you have to leave your computer on all the time. We shut down at night so that would not work for us.
Good luck.