My favourite is something I think I saw on TV and have found to be most amusing. The idea is to see how many times you can say the word "yes" before they hang up.
You: Yes?
Them: Hello, I'm not selling anything, it's a survey. Do you have a couple of moments?
You: Yes.
Them: Ok, If you were going to have new double glazing fitted in the next six months, how many windows would you have done, a) 2, b) more than 2, c) more than 10?
You: Yes.
Them: Sorry, was that a), b) or c)?
You: Yes.
Them: (assuming you must be hard of hearing) Hello? Would you like any DOUBLE GLAZING?
You: (calmly) Yes.
Them: (seizing the chance) Well, can I send around one of our representatives?
You: (Trying not to panic. This might all backfire) Yes.
Them: Well, which date is best for you?
You: Yes.
And so on...

Great fun. A good number of 'yes'es before they hang up is about 10. More than that and you are dealing with a very patient salesperson