How accurate can one answer the call to a complaining guest?

Based on my work as a field service technician, I do encounter a client that complains alot. (ie: in this case a photocopy machine). I just got into the office door and there he goes yelling and cursing the machine is always out of order.... In this situation, I'll just let the client speak out what he has to say and when the timing is right, I will ask him calmly what is the nature of the problem before he called the customer service dep't. Knowing that small piece of information I can quickly picture out  in my mind what is the possible cause of his machine's breakdown. The first thing I will do is try to run test the machine and see if I can also encounter the same problem he told me. Then I will check for visual confirmation what's causing the problem. In this manner, the client quietly observes the professional approach I have showed him that in my long experience in this type of scenario I can tell myself that I am able to calm down my client and I can easily convince him what is needed to be done on the machine to make it work efficiently....