There is a very fine line to trad when dealing customer service for a company that has fucked up royally. You need to get your anger across so they take you seriously, but also avoid being an asshole to the person on the phone who is just trying to do their job.
I don't miss being in a customer-facing role. Having worked in McDonalds in Essex and HMV in southeast London I have had more than my fair share of quite incredibly ride and border-on-violent customers in my face. That kind of exposure really informs how you treat people; I never want to make someone feel the way I was made to feel.