If you ever get called by your bank/building society/credit card issuer/whomever, and they ask you for your security details before they can continue talking to you, remember that they called you and as such you cannot be certain who they are. So, before you answer any of their security questions, ask them to tell you something about yourself that only they are likely to know. That way, you are protecting yourself from potential identity thieves who are trying to obtain security information about you so that they can use them with your bank/building society/etc in your name.
I did this with a store card that I hold. They called me after I left a message with them, and I was about to give them my debit card number... then I realised that I couldn't be certain that it was actually them I was speaking to, so I asked them a security question about me, which they were more than happy to answer for me. It was only then that I could be certain that they were who they said they were.
You are well within your rights to refuse to speak to a company that contacts you, regardless of who they are, if you have any doubt as to who they say they are.