Overpayment of account

I have 2 lines with Cell C!! The one is due for upgrade and today when I drove 140 km to do the upgrade I were informed the account is in arrears!! According to myself I'm paid with at least 2 months in advance but any way!! They claim I neva paid one of the lines for the last 90 days and I own them R780 on the one line!! I informed them can't be I always pay more than the account I should be in advance the claim I'm not!! I log onto the bank in there store and show them proof they belive me and agree the mistake is on there side they will do the upgrade!! After phoning head office they claim they can't assist me with the upgrade!! I feel on the end of the day if the claim they don't link payments I should be in arears for like 24 months bit I'm not only for 90 days how did they then clear the 120, 150,180 and more months if they don't link payments!!! I mean it cost me money to travel to town about R500 for fuel I'm driving a FJ Cruzer plus the time I lost from my work plus the time I spend in there store and on the end of the day to hear sorry they first have to fix there system I can only do the upgrade in 48 hours!! The lady in the Lephalale Branch was very kind!!