Callback and Calling Cards

Negative balance for Calling Cards

A calling card balance will go negative in only one case:

1. A user initiates a callback, and the system dials him back.
2. The user answers and enters a calling card number/PIN.
3. The system recognizes it and checks its card balance.
4. The Card balance is 0.
5. As the calling card is recognized, the charge for Call Leg A is deducted from the Calling Card balance. The Calling Card balance becomes negative. 

This means that system owner incurs a loss. In this case, there is no way of keeping a user from initiating a call and not paying for it.