This is always something a new employee should ask when starting a new job - the response will vary from business to business.
The first thing would be to assess the situation - how upset is the customer - irritated, upset or yelling. If you are working with another employee nearby then get that person to notify the supervisor for assistance. If in another area you may need to excuse yourself for a few seconds to find that person for help.
Best thing is to understand that the person is upset in some way and will not really be listening to what you say. Let the person talk. As soon as you can start talking in a calm manner and in a normal tone of voice - not a raised voice. This should start to calm the person and force him to stop talking and listen.
Beyond that help should come - a supervisor or another senior employee. You go from that point based on your training. Hopefully you will never be faced with this situation but you do what you need to do.
"Sorry for the inconvenience caused. Please be patient for a few seconds, I'm looking towards the problem that has happened and the solution to regain your process."
This would make the customer to hold on for a while, with no more interruptions to happen.
I'd begin by asking what are you angry about.
