Figured a double post ne'er hurts. I think Jim has hit it right on the head.
To quote from above URL.
his is really annoying and could lead to some type of identity theft. I don't know what they really want to do, but they claim they are calling on behalf of your credit card company. After you give them the toll free number on the back of the card, they will know what kind of card it is and then will proceed to give you the first four digits of your card as verification to lure you in. Don't fall for it. Make them give you the LAST four digits of your card. They can't because they don't know them until you supply the information. Anyone that is actually affiliated with your credit card company will already have that information in front of them. Be careful out there. :)