I worked in a call center for a DSL company. This was pretty much a regular occurrence. Probably the number one thing I learned just from overhearing calls from others around me and from listening to training calls is to NEVER, EVER tell someone to calm down. It does nothing but illicit immediate screaming/nastiness. I just let them rant, apologize for the company "dropping the ball", and do everything I was able to do to help. I couldn't always fix their issues but they at least calmed down and thanked me for being so patient with them; most of them actually apologized for yelling right off the bat.