This is obviously if you're bored and have time to kill.
Let them do their entire pitch. Tell them you are now unemployed but really would like to start a career in telemarketing. Tell them how cool you think it is to talk to new people all the time and that you're a "real people person!" Ask them how they got started. Ask them how much money they make. Ask them if they could talk to their boss about hiring you. Don't let them get off the phone until they hang up on YOU!
OR
When they ask "How are you today?" Tell them! "I'm so glad you asked because no one gives a shit these days! Life sucks! I have all these problems; my hemorrhoids are acting up, my dog just died, my girlfriend just told me she had vaginal warts, etc."
OR
If they want to loan you money, tell them you just filed for bankruptcy and you could sure use some money. Tell the telemarketer you want to apply immediately but not over the phone. Tell them you are on "home incarceration" and ask if they could bring you the paperwork, a case of beer, a pack of cigarettes and some chips.
OR
Insist that the caller is really your buddy playing a joke. "Come on pal, cut it out! Seriously, how's your mom? No matter how hard he tries to convince you that he's not your friend, keep grinding him and tell him YOU KNOW it's him and he can't fool you!
OR
Another great time waster is to tell them to talk VERY SLOWLY, because you want to write EVERY WORD down. No matter what they say next, ask them to repeat themselves over and over again. Ask them to hold on over and over again too. Just put the phone down for 30 seconds to a minute each time.