I saw Seinfeld episode once where a call center phoned up Jerry, he picked up the phone and,

"Hello?"
"Hi there sir, would you be interested in......."
"Well, I'm busy right now, so how about you give me your home phone number and I'll get back to you"
"Uhhh, we're not allowed to do that sir"
"Oh, I suppose you don't want people bothering you at home, right?"
"Well, yes"
"Okay, now you know how I feel"
Hangs up phone.

I found that rather clever. I get a billion calls a day too from call centers. When I see a 800, 888 number, I just pick up the reciever and hang up again. They keep calling and I've told them to bugger off in a mean way. They stopped for like a wekk, but, they're back at it again.