You made me pay for my calls twice!

On my January invoice I was charged for domestic calls (\Calls Category 1\" on my invoice) over December, while I already paid for that month in the beginning of December because I was on a Smart M package. I definitely did not go over my minutes!! (I normally never do, and now I was out of the country for most of the month, so I am sure about this). The problem probably occurred because I was on a Smart M Package, and then \"upgraded\": got myself a new phone + a new contract on 1 December, which is also a Smart M Package, and somewhere something went wrong on your side. You can go back into my invoice and statement history and you will see that I always paid my Smart M Package upfront. So you owe me money! By now, over the past month (ever since I received that January invoice) I have spoken to about 5/6 Vodacom people. Of Customer Service, of Accounts, and the last one being a manager. They all promised to sort it out, but it still is not. The manager that I spoke to 3 days back promised me that the problem would be sorted within the hour. So Vodacom, when are you going to repay the money that you stole from me?"