Um, I'm not sure who is calling you but I know for a fact that it has always been Wells Fargo calling us. They give us more than enough info to confirm they are in fact with the bank and they have never once asked us for a card number or for our social security numbers. So while this is good advice in some cases, it's not if you actually have a card with the company calling because I have no doubt that WF would cut off our cards if we didn't verify our use of the card.
The safe thing to do is hang up and make a call to the credit card company. This happened to a relative. They verified the charge was correct. Also told her she did the safe thing. If it was a fraudulent charge the card would be cancelled immediately.
I have had the company call on several charges I made in other areas. They told me to call them back at the phone number on the back of the card. It was quickly resolved.