Whenever I see a telemarketer or "unknown" on caller ID. I just click the answer button on the cordless phone and set it right in front of the stereo speaker, so whether it's jazz, or classical, or dance, or rock, or TV that's on the stereo at that time, that's what the caller hears.
Like eternal music-on-hold!
Usually when I check back in 5-10 minutes, they've hung up and there's a dail tone or dead line on the other end.