I try and think of the person at the other end of the phone as me..Human and with flaws and just as likely to get flustered by someone who is all stressed themselves.People who are left to flounder because they have had inadequate training,their supervisor is a dragon to them if they bring her/him any problems and who are doing a tedious,boring,stressful job because they want to be helpful and need to earn a living.
You wouldn't want to be unkind to yourself now would you?