As mjevans says above, it's fine if they tell you there's a problem, but you should never try to solve that problem over the phone based on a call by a stranger.

Unless you know exactly what you're doing, what you should do in the case of any over-the-phone problem like that, is first contact someone you trust and knows what they're doing. Your own bank, an expert of your choosing (and not chosen by the stranger on the phone), or a knowledgeable friend or family member.

Never trust the stranger on the phone.