A2billing low balance transfer to extension (reposted)

Hello, Basically I need my a2billing modified to transfer the caller to a certain extension when the balance is too low.. So instead of saying "Your balance is too low to complete the call" it would play a wave file and then transfer to an extension to speak with an agent to refill their account. I need the code and instruction on how to install it... Note: Since this is a live production server, I will not be able to give you access to it, You will need to work on your own server and give me the code when done. Thanks