The best thing to do is to keep them on for as long as possible, by either feigning interest or talking nonsense, with no intention of buying their useless trash.
Make a record sheet and see how long you can keep them on for (my personal best is 2 hours 15 minutes)
You will no longer dread the calls but see it as an opportunity to beat your personal best. Also you will be gaining a modicum of revenge on the telemarketers by waisting their valuable selling time.
I actually had one rep crying once because i had her on 50 minutes before she realised i was winding her up, (i think asking about her pet turtle gave me away)