I have dealt with a lot of challenging clients throughout my life, and the thing that I mostly do is I stay calm because I know that they are agitated, whatever the problem might be. I stay calm. It's important to use a calm voice so they can mirror you and calm down, validate their feelings, repeat what they're saying so they know that you understand what the problem is, and once somebody feels understood and knows that you can handle the situation in a calm manner, they usually calm down.