The reason they'd ask you for your details would be for in terms of data protection, and to ensure that they're definitely talking to you.
I make it my policy to never reveal details to a cold caller. I have no idea who I'm revealing my details to. Besides that I don't pick up unrecognised numbers. I should like to state my annoyance at being frequently called by this number (0345 112 6604). I accept that on checking I seem to have had my preferences set to allow calls, although I don't remember ever allowing it in the first place. I'm very careful about these things and usually refuse any option of marketing through the phone. However even if I technically agreed to calls, calling me every single day, sometimes several times in a day and at unsociable hours is to me a nuisance call. The calls go to my answer machine, just leave a message if you want to discuss packages and that's the end of it. I've set my preference to disallow the calls now, but obviously I'm still getting them. I understand it does say it may up to 28 days, though I don't find that acceptable either. Why so long to process this? I've been happy until now with Plus Net's marketing and have never had marketing phone calls. Only emails.