Everyone trying a new billing system should try the following test:
1) Place one call ,on prepaid mode. What will be the normal prepaid calling card operation:
When the first call is in progress, make a second call using the same account number: You guys will be surprised how many systems could be tricked that way to give you two $5:00 dollars calls on a single $5:00 dollars card.
Matter of fact, unlimited ammount of calls could be done that way unless one of the callers hung up and the ammount gets deducted from the balance.
2) Second test: Make one call on prepaid mode like before.When you finish the call, dial a second call using a dialer or speed dial from a phone with memory.
Some billings will take few seconds to access the DB and deduct the ammount of the previus call from the balance, allowing a second, very fast call that will get the full balance, like the first call never was made.
Oh, yeah. I read all this in a book. Never happened to me.
I will not name companies, but this troubles are present on the billing system on the list.