A2Billing Callback Project

I'm looking for someone who has experience with Callback on A2Billing. My current implementation uses the built0in A2Billing Callback Daemon, but this does not work well for high volumes as both legs of the call has to wait until they are connected before it moves on to the next call in queue. I need a tweak or solution that will allow the system to handle a large number of calls without significant delay. Please contact me if you have specific experience in Callback with A2Billing. Also, my current budget is limited, so please keep this in mind. Our service is not yet launched. Thanks in advance Richard