We have a few articles that explain how to set this up. Here is one. We recommend searching our forums, the answer to your questions are normally here.
AT&T Customer Care
Need help with an account specific question? Post a new question here on the forums by clicking the "Ask a Question" button. For additional support, please visit us at our AT&T services hub. Follow us on: Twitter @ATTHelp
*I am an AT&T employee, and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.