Actually a pretty good idea. I got pissed one evening after 5 telemarketking calls, so I went out and spent the $50 for one. It's worked, but chexi's idea would do the same thing. After it's hooked up you get a lot of hang ups, and that means it's working. The good part is that once the computer thinks your number is disconnected, it spreads to other databases as such. I haven't gotten a single telemarketing call in 3 or 4 months. The only down side is that it beeps every time you pick up the phone. Friends ask "why does your phone beep like that??" and it creates a 3 second silence before my answering machine message will play, and even though it's only 3 seconds of silence, I've had some impatient people hang up and tell me later that my machine has no message. "you didnt wait long enough"
Some phone companies (Qwest for example) offer services which will screen telemarketing calls. Any call that is "unidentified" will not ring until the caller leaves a message stating who they are, and then you have the option of taking the call or not. Kind of a pain, but effective 100% of the time. I think it runs like $10 a month though.
Hope that helps!