Someone to whom you should probably be nicer if you're not already?
Anyone you talk to when you call customer service. Especially if you're calling a large business like a health insurance company, hospital, major bank, delivery service, etc. I get it; you're mad as hell and not going to take it anymore.
But keep in mind: the person you are talking to probably sits behind a desk in a windowless, privacy-less, uncomfortable room. The type of room that has insincere "teamwork" posters on the wall and a nauseating gray/taupe color scheme. What's more, chances are, they're making a crappy hourly wage and grossing under $30k a year. (IE, not enough money to be your punching bag/therapist.)
Most importantly: there is a 99.999% chance that the person with whom you are speaking is NOT responsible for your grievance.
That person--the decision-maker or source of error--doesn't get your phone call. If you're lucky enough to actually speak to that person, sure, let them have it. But I'm guessing that you won't ever actually talk to that person. In the meanwhile, be nice to the other people you encounter.
Kindness matters. I'm not saying your anger isn't warranted; I'm saying that you should check your hostilities and make sure they're not misdirected.
P.S. I'm not saying that the people up top don't work hard/haven't earned their positions...I'm just saying that they're not the people you encounter when you call. For what it's worth.