I'm hesitant to call back any of these people, though your idea of calling them to tie up their line sounds good but if I were to call back any of these nuisance callers, would not they be able to claim me as a client?
I ported my land line number to my cell phone. So now I save the number into my contacts and and assign then the ring tone of silence. They will never be heard from again! Each contact can have 4 phone numbers.