First and most important is going to be your Body Language/ Initial Response. Maintain eye contact, Listen, Listen, Listen, Understanding (yes) nods. Respond with an empathy and acknowledgement statement such as" I do understand why you are upset, I would be too, initially". Then follow up with an " action statement" such as " My top priority right now is you and I am going to help resolve this issue". Reassure them. Be sure to talk clearly and at a reasonably soft, mild mannered volume and pace. This (should) help bring his/her emotions even to your manner. If not go back to step 1 and Listen more until you can achieve some degree of calming the situation. Ask questions that are pertaining to the situation if you need to in order to understand. Up to this point I would have maintained a seriousness in my behavior, and now after letting them vent some I would begin to show more smiles and follow through with my statement of resolving the issue with whatever protocols I have at my disposal. Begin to build some rapport and common ground once you are making some progress in the resolution process, ultimately leading to an end result that leaves the impression on the customer that I was a very helpful and an effective problem solver...and that Enterprise is a class act outfit to continue to do business with in the future.. Customer is happy with outcome and Thanks Me!