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