God knows! I was too pi$$ed of to bother asking...
I usually use that tactic to get rid of people. I just ask them where they got the number
Them - "You completed your details on an Internet site"
Me - "Which one?"
Them - "No idea - our computer system does not say"
Me - "Ok then - take me off the list and don't call again"
Usually does the trick. This one was probably a MyOffers.co.uk competition entry thinking about it...