Use relaxation techniques and breathing exercises to reduce stess. Simply breathing very deeply and releasing very slowly has a calming effect. Simply closing your eyes and repeating a calming phrase for a few minutes can be very calming. Seek out support from collegues and supervisors. Talk about your problems.
Chances are that co-workers have the same problems.
They may have good ideas on how to deal with them. Just talking with other people makes you feel less isolated and puts problems in perspective. The way you are perceived over the telephone is very, very important. It affects your ability to diagnose problems and help customers. It has a profound effect on your total work atmosphere in technical support.