Incorrect billing

In November month my fiance and I made a deposit at a ABSA ATM. We deposited a R 1000 into our telkom account using the ref number we were send on a sms. After everything was fine we threw away the deposit slip as we have done the depozit before and everything was fine. Yet on December's bill it shows we still owe them that R1000 we deposited. I've read a lot of comments on how to consultant will see your deposit on their screen and still tell you there isnt any payment reflecting. I just have a feeling this is going to be my situation, and I will be ****** of my R1000