Listen to them and let them vent. Sometimes they need to do that first. Find out what will make the situation right in their mind. Most importantly, you need to care. Empathy. Even over the phone, a person knows if you are just going through the motions. I believe that I am the voice of the company and I don't take that lightly. Without that person calling and complaining, the company/I have no way to right the wrong. It doesn't matter if it's only the customers perception. It's their reality and it needs to be validated. Without that customer, I have no job.