My general rule is that I pass no information whatsoever and refuse to engage in any way with people who cold call. They may seem legitimate, like my bank, a charity, Sky TV, whoever, the rule is, sorry this is not appropriate. I do not buy, order, confirm. If I want double glazing, I will seek a company of my choice. If I want to reduce my utility bills I will do in under my own volition and not as a request from British Gas or NPower. I'm not interested in prizes won from a competition I haven't entered and I dont want to do a survey.
I am usually polite but very firm and don't give them time to respond. I have been rude on occasions (especially with a Sky TV operator who wanted to convince me to "come Back" ) and a few times have had a bit of fun with the pc scammers. Just refuse to engage with them. Say thanks but no thanks and hang up !
Its time for tougher regulations. None of us require cold calling. It's not essential to us in any way and only serves the companies that condone it, even the legitimate ones. It's a nuisance.
The practice of selling our personal information needs to stop