I get a dozen calls from these idiots a day. only thing is that i don't ever have to answer a single telemarketing call.
i have an PbxInAFlash box (http://pbxinaflash.net) that sits between my pots line and my phone.
When you call, your caller id is looked up against a shared list, much like rbl, and if its blacklisted you get disconnect tones and hang up.
if your not on the list, and you havent been white listed, you must still prove that you are a human by dialing an extension number that changes per call. If you prove your a human by pressing the right extension, you get to talk to me.
There is however always an extension that announces as "if you are a telemarketer please press 1 for immediate service.
pressing 1 or failing to press anything, dumps you into telemarketer hell. an announcement of "your call is important to you, please hold and your call will be ignored in the order in which it was received."
then you get elevator music for a while, and another announcement comes on" please continue to hold, you are number (some large random number) in the cue and the estimated wait time is 99:99 minutes"
evil, but highly effective