Seems like the correct response is no response because you don't want to confirm that the number is in active use? They must at least record a bit that you replied, which makes your number a more valuable commodity.
1) answer
2) go mm-hmmm to their first question
3) phone on mute and set it on the table and go about my day
I think the record for a telemarketer monologue is a bit over 3 minutes before they figured out I'm not responding. That was 3 minutes they weren't scamming anyone else to subscribe to whatever. I'm practically a human Tarpit for telemarketers =)