So, there was no billing error here. The guy actually had his modem making long-distance calls for inordinate amounts of time. Doesn't seem like an AT&T error. Though it definitely sucks for the old man/woman!
No billing error? The entire billing system sucks balls at the largest possible frame.
There should be a legislative directive that all such usage-based billing plans provide an option for the end user to set hard spending caps, which are automatically enforced by the service provider.
Show me a corporation that doesn't—at least attempt—to enact hard spending caps enforced by automatic systems wherever and whenever possible. Heads roll in the gutters when a corporation loses $100 million because some trading desk manages to go rogue with respect to set trading limits. (By the Finnish system of traffic fines, a $100 million loss for AT&T is about on par with some old geezer tabbed for $25,000.)
End users are, of course, purposefully disadvantaged to have to police their own usage by manual vigilance, because everyone knows this is a lucrative fail mode for AT&T's revenue piracy service.
That this whole thing sucks balls right down to the bag root is the least possible diagnosis.