Collecting Digits Through Prompts (before prompt playing is finished)

I installed Free PBX with a2billing recently and everything seems to be working without problems. However, I cannot find a way to set up the system to start collecting digits from customers before the promtps are finished playing. I would like to be able to collect digits (PIN NUMBER, Destination number, etc) before the voice promts are finished playing. If anyone knows how to set that up please post the instructions here. Thank you!