Questions about invoices

I find it fascinating how access (lines/trunks) providers bill for services to their customers, especially when the bills have absolutely no detail. What questions do customers ask when paying almost 2K a month for service and almost one half of the bill is a credit? How can a relatively small customer be given a 45% discount. Seems like something was over priced to begin with right? Questions, questions, questions.
Oh yea, that thing called smoke and mirrors hmm, just wondering about those discounts.