Don't take it personally
I was working as a Secretary for a Government Education Department. The other Secretary in the office was the Customer Service Director's assistant.
I heard her reply to a phone call from an obviously disgruntled student. She had no idea how to handle this conversation, she got most upset and angry with the caller. Her comments were like "there's nothing I can do about that", "that is our policy I don't have the right to change it".
Very personal comments as if the caller was unhappy with her whereas he was unhappy with something about his training course.
She was quite upset when she hung up the phone and I'm sure the customer was even more so.
Two important lessons I learnt from hearing this conversation:
One - train your staff on how to react to these calls.
Two - don't take it personally, the client has a right to complain - let them vent and try to help them if you can. If you listen, they'll usually calm down and a resolution can be made.