A simple method to reducing anger is to be prepared and consider going higher in the chain of command.
So, if one has a dispute with some service provider then simply let them know and if it is not satisfactorily resolved then you simply ask for the management. The first thing that this person is likely to say is “she will just tell you the same as I have”. This has happened to me a number of times, and is the person’s way to defend their position. Simply repeat that you would like to speak to the manager or get their contact details. Then once you speak to the manager and still have not resolved the problem satisfactorily, then once ask again to speak to their manager or owner or boss etc in the chain of command. Never give up and this will help you to refrain from getting angry.
It my take up more of your time initially but the pay off is we learn to keep our cool and gain more quality time, piece of mind and control.