In our line of work, we deal with customers who are not just upset but downright furious now and then. Seth Godin, author of many books on marketing, has some great advice on how to deal with those who would like to take your head off. Here is a highlight:
"Talk more quietly and more slowly than the person you’re talking with.
Not an exaggerated mantra, but just enough that you will be
de-escalating, not escalating."