Friend of mine, years ago, in a job that beyond the fact he used a computer from time to time had nothing to do with IT, had a particularly grumpy customer on the phone, and was getting angry, so needed to compose himself. He put the phone on hold (or thought he did), making an excuse he needed to check something with his manager, put the receiver down, then said "Fuckwit" ,only to hear a tinny voice on the other end of the phone say "Who are you calling a fuckwit?".
Not sure if the customer complained, but that is almost irrelevant as his manager was standing in the office, and asked to have a polite word with him.
I never found out exactly what was said, but he left the firm a few weeks after, deciding to move back to retail (which, in fairness, he was more comfortable with anyway).