I go for the shock value.
I usually let them go through their spiel then ask them if they are married, what color panties they are wearing, or I will put the phone down and have my daughter let off a couple of blood curdling screams while I yell in the background pretending to be smacking her around, then hang up.
Asking irrelevant questions like (Do you walk to work or take a lunch?), works too, but it's not as fun.
It horrifies some of the people on the other end, especially if they are calling from overseas.
Good times....