About Negative Balance

I am using A2Billing 1.9.4 (Cuprum) version. Its working good, But at this time i have a problem. I had started 2$ and 5$ calling card by using A2Billing. I have configured a destination and used rounding calltime, rounding threshold, and also additional block charge, additional block charge time. Before using those system ( rounding calltime, threshold, additional block charge) the two dollars card had finished in 20 minutes, but i thought that this time is too much, so i wanted to decrease this time by using rounding calltime, threshold and additional block charge,but customer will see that they has called 20 minutes. Now When i set those system then cards are using the same duration(20 minutes) and after using a card, its balance show -0.68 $,
Now Please send me a solution that how can i stop this negative balance and reduce 20 minutes to 16 minute for one 2$ card.

Better you ask this in A2billing forum. forum.asterisk2billing.org/

–Satish Barot
Ahmedabad, India