I am really sorry to hear that. Let me be the one to help you. It's never happened like that before. It's not our policy to do anything over the phone. You have to write, fax or come in personally.
Some words are triggers for angry conversations.
Explain what steps you will take, and when you will get back in touch with the results. Get the problem fixed. Resolve the misunderstanding. And when you do fix the problem, go the extra mile. Give them a bit more than they expect.