Also, as someone who has worked on the technical side of the a large telco's billing department; I can guarantee that it was in fact a mistake and not malice. Our billing system constantly had errors. We weren't as large as Verizon, but we did have to issue refunds of maybe $100,000 every couple of months. It was usually do to some regulation and taxes being improperly applied. Telco billing changes so much so often it is hard to build a billing system that can change to any and all ways the government decides to regulate and tax it. Also if you add a new switch to your network it can log and parse billing files differently causing an error similar to what is being described by Verizon.
So as someone who as actually worked on Telco billing systems, I'm on the "it was an accident" side.