When dealing with an angry customer whether right or wrong, we have to first listen to what their complaint is and really understand it. The best practice here is to listen and then repeat back to the customer what you heard. Keep doing so until there is agreement on the problem. If they are upset let them know you can tell they are upset. Keep your tone calm. Empathize, do not sympathize. If someone on your side screwed up, own it and if amends are needed, make them. As business people, we are dealing with value propositions when dealing with our customers and if they are upset, a temporary value adjustment can help smooth things over. Also, by giving up some margin to help calm the customer, we learn to do better so we can preserve our margin through better customer service.