I wonder if, perhaps, this is a two's complement problem. The phone company actually owes him a slight amount, but the software misinterprets a negative number as a high positive number. While computers use two's complement numbers, we can visualize the problem by using 10s complement, as on the odometer of your car. If you start at zero, go forward 1 mile, then back two miles, you've gone -1 mile, but the odometer shows 999,999.
Harold