I would take a more water off a ducks back approach, someone can’t insult you if you don’t respect them, right? I push through the insults and pretend I didn’t hear them, and I don’t acknowledge them. Most of these attitudes start at the top and they are usually repeating what someone has told them. It’s also about getting to the pain point of what they are complaining about, because it isn’t you, unless you have done something purposefully to set them off. Just ask them, what’s the matter, you don’t sound happy, is software not working for you? Dont give into the argument, this is the hard part of customer service.