One of the first things that you need to remember, when you come up against a rude client is “Don’t take it personally!” What it comes down to is the reason they are being rude may have nothing to do with you at all. It could just be a side effect of something else either in their personal or business life. By taking it personally and acting accordingly, you my just be making a bad situation even worse. One of the best things you can do at this point is to just listen.

I agree with MS,: Stay professional.

answering on my personal exp. first you need to listen calmly and with full concentration, remember if the other end person is rude / angry then that person definitely makes mistakes during the talk. you need to catch those points and discuss with client stating "listen i am asking you few queries to understand the situation & let us conclude on the middle point ' during talk you can calmly put the client on back-fut with the points having you & settle the issue.