asterisk users - Jul 2005 - Limit Minutes / Mensual?

Hello,

How can i create a plan who limits some users the length of his calls? 
like a limited mensual plan.  I think that option have a special name, 
but, i'm newbie and don't know it :(.
Please help me.

Note: If anyone, need help with Cisco AS5300 and Asterisk, ask me!! i've 
set it up, and It's working.

Thanks