devious17,
Many call centers have recorder systems that records conversations. Those recordings are somehow, "valid" endorsements. If you mentioned that Citibank is charging you for something for months without you realizing it, you are no longer entitled to make a complaint anymore. Reason is because they send statements every single month. You should've made a report once you see the very first "unauthorized" charges made to your account. After a few months, they would have already confirmed that you agree with the transaction.
However, I would still encourage you to fire it up with them and see if they can prove anything for charging or not. Better to fight and lose (if this happens) rather than loose without fighting. God be with you, dude.
Back to the topic, I've got calls from people who I don't even know. When I asked Maxis about it, they said that it could be from wherever forms which I filled in in those contest or whatever.
Another possibility is that some of your friends were registering something and one additional section was to include some referals for, perhaps, a little more benefit of whatever they're signing up for. There you have it. Another way your personal details was let loose.
There are many more ways, I believe, where we're somehow not entirely aware of on how our personal details are distributed. Scary when I think about it. Can't even read a magazine while shitting without someone calling up later and say, "Sir, I'm from fuckYOU Company. We noticed you were reading HOW TO FUCK A TELEMARKETEER magazine in the gents just now and.... bla bla bla"