We are on the DNC now so we never get them.
When we did I used to ask them stupid questions, what are you wearing? what are you doing after work? are you going on holiday this year? what's your favourite ice cream? have you got a rice maker or do you just use a pan? etc etc
Sometimes I would just give the phone to my youngest daughter.
I do sometimes get calls from the bank and they ask me to verify who I am. I always say - You called me, what's your date of birth?