why pretend!?
Better then actually fapping while you're on the phone.

why pretend!?
And if you're not actually registered then register - it's free http://www.mpsonline.org.uk/tps/. Do the mailing preference service registration too - it takes a while for the junk mail to reduce but it works.Next time they call tell them you are registered with the telephone preference service & if they ring again you will report them.
Pretend to start fapping whenever they call
If you are with BT, they offer an anonymous call rejection service which is £4.10/month
http://bt.custhelp.com/app/answers/...anonymous-call-rejection-and-how-do-i-use-it?