There is a free service called NoMoRobo that implements a massive cooperative blacklist on a grand scale. I use it on my Comcast phone (requires multiple ring). One of the few workarounds for tele-scammers is to falsify caller id with a random number in the victim's area code and exchange. Most telemarketers who call me are dumped by NoMoRobo after the first ring, but once in a while I see what appears to be a local call from an unrecognized number. Any number I don't recognize ends up in voice mail, which is where telemarketer calls go to die.