Callshop billing system

Hi, I am looking for a callshop billing solution similar to "Trixbox 2.0 / *starShop 0.3" I presently have : "VMware Trixbox 2.0 / *starShop 0.3" -Main feature I need are for the agent to be able to lock and unlock phones -have credit limit -have a total at the end of each customers session for the agent to charge the customer I will be interested in a simple system which can be set up at a low cost with my own voip providers. Thank you