If you're the type willing to spend time messing with them, consider adding this to your arsenal:
If you have Callcentric or another VOIP provider, you then have the option to create call treatments for forwarding a good percentage of telemarketing calls to any number you want, including the telemarketers themselves.
For example, one of the ways I get target numbers to forward to, is by responding to the Google SEO guys then pretend to be cut off mid conversation. When they call back since they think they have a good lead, the caller ID (surprisingly) is almost always a valid number to the call center. That's the target number. Even just faking an emergency and asking for their number so you can call them back usually works. Once you have that, Bob's your uncle since there's not much reason for them to change their block of unpublished incoming numbers.
Then it's simply a matter of going into the dashboard, creating a forwarding treatment of all obvious caller ids (i.e. any 800*, anonymous, +1, etc.) to the target number and voila, the call center gets hit with all my forwarded telemarketing calls transparently. And of course forward the target number back to itself, or even better, another target.
The best way is if you can whitelist your incoming calls and simply forward any non-matching numbers, especially since most telemarketing calls these days use a random out-of-area code caller id number. Not realistic if you're running a business but for personal lines you can whitelist the area codes you might expect valid calls to come from.
Obviously this doesn't work all the time. But when it does, it's pretty satisfying to check the online report at the end of the week to see all the forwarded calls that transparently went to Raj and Rachel. My way of paying forward the opportunity to lower their interest rates.