Becoming upset is OK, becoming angry is not. Be able to explain how you keep composed, understanding that in a TSS role that you are responsible for helping to solve customer's problems, you didn't create them. Being able to provide examples where a customer really got you upset, and how you dealt with it can be used here. Specific examples of how you diffused the tension, have any games you use in the team, tools that are used to 'lighten things up' would be good.