Generating a correct invoice is a monumental task for every carrier. The rates could be wrong, the monthly charges could be double billed, or the carrier could be billing you for features you never ordered. That list just covers some of the aspects your carriers have control over. Taxes are another issue and generally can’t be explained without some shades of gray, even by the government organizations that administer them. Check your phone bill every month against your contract to ensure that every rate and charge is correct. The likelihood is high that over the course of a year, you will find at least one thing that is wrong.