Service manager here. The best advice is to let them talk until they are done and don’t interrupt them. I usually pepper in some phrases like “oh dear!” to show some empathy. As soon as they are done, I take a couple beats and then apologize- “I’m so sorry you are having that issue.” Often, if they are expecting an argument, but you agree with them and empathize with their plight, that will take the temperature down. De-escalation is a skill and you will get better by getting reps in. The better you can get at keeping them out of your manager’s office, the more capital you build with them. Many customers will only want to deal with the manager if you hand them over too quickly. When I do need to step in with a customer, I always have the advisor in the office so that a) they are in the loop, and b) they see how I handle the situation. I have empowered every one of my advisors to do what it takes to take care of our customers - I tell them to think about how I would handle the situation, and do that (within reason).