Leave them talking
I had a problem with cold calling on a BT landline. I tried abuse, it didn't work, they are made of stern stuff these cold callers, I asked not to be contacted but they just got someone else to call who would say "oh, I’m soooo sorry, they haven't put into the computer that you don't want to be contacted, but whilst your on the phone" argh. so I came up with a cunning plan, rather than get stressed out about it, I would answer the phone and as soon as I realised it was a cold caller I put the phone on the arm of the chair, left them speaking to Mrs Thin Air, while I went off and did something far more interesting. It seems to work for me because the calls do eventually stop; after all they are wasting their time and money, not mine.