First, de-escalate

It's very difficult to reason with someone if their hair is on fire. Customer service (whether you're a school principal, a call center or a consultant) can't begin until the person you're working with believes that you're going to help them put out the fire on their head.

Basic principles worth considering (are you listening, Verizon?)

The first promises kept are hints that you will keep future promises. Putting people on endless hold, bad voice trees, live chat that isn't actually live, an uncomfortable chair in the waiting room, a nasty receptionist, unclear directions to your office, bad line management... all of these things escalate stress and decrease trust.