Hi, i found solution but i depends what are you trying to accomplish.
If you need customers to signup for prepaid or postpaid account then use signup form. ( Either prepaid or postpaid customer is defined in one of the files in signup folder). How to set up a reoccuring charges - i don't know.
If you are selling regular calling cards - you can use API call which basically calls a2billing from checkout_process file. It works perfectly if customer buys one calling card. But if he buys more then it gets tricky. This is mentioned in that forum you discovered.
can you do pin generation on fly. if you can please post your solution.