I'm sorry you had to go through that, but unless you work at a suicide hotline or one of the departments that tracks down child-porn creators or something along those lines, people breaking down at least once a week should not be "unavoidable." I know help desk is always going to be stressful, but employees need to be empowered to speak sternly to, and hang up on, abusive customers. If they're not, that's a management failure, not just an unavoidable reality.