Positive Attitude: whether you deal with your face to face or over the phone.
Listen: Let them vent because sometimes that's all they want.
Patience: Sit back and take a deep breath so that you can remain calm no matter what is going on around you.
Accuracy: It never hurts to look over your work and make sure it's correct.
Help: Assist your customer in any way possible that's within your limits.
Time Limits: When you need to call back a customer give them an approximate time frame. If the time comes and you haven't been able to get the information they need. Still call them to let them know that you're still working on problem and will call back ASAP.