Callback and Calling Cards

Negative balance for Calling Cards

The balance for calling cards will go negative only in 1 case:

1. When user initiates callback - system dials him back
2. User answers and enters calling card number/pin
3. System recognizes it and checks it's card balance
4. Card balance is 0
5. As calling card is recognized so charge for Call Leg A is deducted from Calling Card balance. And Balance becomes negative for Calling Card. 

That means that system owner gets loss.

There is no way how to avoid that user initiates call and does not pay for it.