Remember the angry customer is really a nice person, and has temporarily become a sheep in wolf’s clothing. Think of them normally reasonable, and in a good or neutral mood.
They’ve probably called you before with a routine question, and been okay. Now you’re experiencing a blip in their behavioural radar. When talking to them, remember there’s a nice person in therein and if you keep your cool and work with them, you’ll discover that nice customer again. Typically they’ll apologize and thank you profusely if you keep thinking they’ll become nice. Endeavour to treat your customer well , don’t ask for their wrath