asterisk db balance check before sending the call to a2billing

I use asterisk/a2billing for my SIP/voip customers to make international calls. I would like to apply the following logic (with the agi-conf is set: answer call = NO): to build some dial plan before launching A2Billing to check the customer's balance, it would need to be authenticated (SIP account name) before checking the balance. If the balance is greater than 0 send the call to agi-conf2 (go ahead with the call), if not, send it to [login to view URL],9 (an audio file will be played). This could also be achieved by writing an AGI script in php. Please note that I do not want to answer the call to do this check, only if the balance is 0 or less the call will be routed to [login to view URL],9 which will answer the call and play an audio file. Currently when customers dial 200 they are routed to [login to view URL],9 where they can find out their balance. Marcin