What we were always advised to do with cold callers, especially if you are registered with the TPS, as most people seem to be now, is to ask who is calling, get a name and a company and a contact number. tell the caller that you keep a call log of cold callers for the TPS as you registered with them on whatever date you registered. If they then call again you have the info of their last call, and when they were last told not to call you again. Its a little bit of effort but it can pay dividends if you are being repeatedly called by a particular company.
We often just ask them if they have accepted Jesus as their personal saviour. Somehow, they never want to hang about on the phone. Shame really as I have been known to teach at one of our local Sunday schools and know the New Testament pretty well. One of the Pastors at our church suggested it and it works a treat. Our Pastor has a strange sense of humour. It works, they rarely call twice.