How to configure trunk in a2billing

I am sorry to post this message to here.but I cant find any message in aah forum which can help me solve the trunk problem.
I have configured a trunk in AMP and it works well.When i need the user be charged because of calling,so i must add users by a2billing,but i dont know how to configure a trunk.The add trunk in a2biling is not very clear.
Can someone help me ?thanks.
Best regards.[/code]

Anybody can help me ?Thanks