One-Shot-One, I've used a variation on that one as well.
Sometimes, if I know it's a telemarketer, I'll talk like an old man.
"Ooohhh, it's so good to hear a voice. My kids never call me anymore. Even my oldest son. You know what? I think it's his wife. She..."
"Sir, we're have a special today..."
"Special? Talk about specialists. I had one do a colinoscopy. You know what he found?"
After a minute or so of this, they hang up.
Another fun one, if I have the time, is to sound like some sort of generic Arab type.
"Hello, Mr. Baker?"
"Is Abhib."
"Excuse me?"
"Is Abhib. Becher Meat Market. We have special today on bif. $2.99 a pound. How much you want?"
"No, sir, you don't understand. I was calling about..."
"We can even sell you whole side of cow. $59.99."
"Mr. Baker, er, Abhib, I was calling about..."
"Ok, I throw in delivery. Soon as brother gets back with car. We have lamb, too. You want lamb?"
Eventually they all give up.
One that didn't go as expected was as follows:
"Mr. Baker?"
"Yes."
"Congratulations!"
"On what?"
"You've just won free dance lessons at Arthur Murray dance studios!"
"This is a joke, right?"
"Oh, no sir. You've won a free lesson..."
"Bill put you up to this, right?"
"Mr. Baker, this is no joke."
"Well, then John put you up to this. Whoever it is, it's a sick joke."
"Why?"
"Because I lost my leg in a motorcycle accident ten years ago."
There was a few moments pause, and she came back. "Well, sir, is there anyone else in your family that would like free dance lessons?"
Have to admire her tenacity.