No apologies
If you work in the service industry or as a customer service representative, you are probably trained to know this already, but if not, remember it well. Never apologize for a mistake that is not yours. Instead, say things like ‘let’s try to focus on the problem’ or ‘I understand your frustration’. These phrases will diffuse most people who are already heated.
You want to give them the feeling that you understand them and that you are on their side of the argument, working together towards the best solution. Never apologize in general for mistakes that are not your fault, you are giving that person too much power over you in any situation.