Well you could buy a call blocking device. They're available online and start about $40. You also could check with your local telephone company and see if *60 is available. Here is some more information for your reading pleasure. http://800notes.com/nb/search?q=call+blockers
Double check your comcast. You should be able to dial *60 to access call screening. press 2 to add numbers up to 25. There should be no charge fir this.