Comcast will only block some "unknown numbers"; all the others are allowed thru.
The only way is to get a call blocker that will block all the unknown and out of area numbers, it also will block spoofed numbers and you can all up to 80 numbers to block.
The one I have I got on Amazon called the Digitone Call Blocker and it runs a $100. There are other ones that use a computer program and the computer must be on 24/7.
I see that my blocker blocks about 3 to 8 calls a day without any ringing, I just know when I look at the blocker and it will tell me what numbers that have called.
So don't count on Comcast for any help.