I have had experience of my bill being wrong and it taking several calls to get things sorted. My problem was in relation to a price increase and discount which was agreed but was not applied. Eventually, it was sorted. The explanation given was that the initial discount was agreed but not authorised so was not applied. It needed a higher authority to authorise and this was not initially completed by the agent. I have subsequently had a false notice that my account was overdue (discussed elsewhere on this forum) which has also taken several calls initially to make contact and to sort out.
Over the last 12 months, I've had issues with many companies (including Virgin's direct competitors) whose systems have failed or have staff who do not understand their systems well enough. It is unusual for these companies to have easy access to solve these issues. Mostly, it is a phone call to a menu and ultimately to an overloaded customer services centre with a long wait accompanied by music I would rather not hear. Despite all the 'passion' to provide excellent service, few companies actually look act things from their customers point of view and deliver excellent or sometimes acceptable service.
Keep ringing until you reach someone who actually understands you and your issue. You may then have a chance of resolution; unless, of course, a kindly forum moderator comes to your rescue and provides a short-cut.