MY ACCOUNT WAS DEBITED TWICE WITHOUT MY CONSENT

I had recently taken a cellphone contract with Telkom, my first debit was this month however I was debit twice. I was debited an amount of R498 which I knew about and signed then later on another amount of R6819 that I never gave consent for was debited. When I went to the branch I had taken the contract at I was told only the R498 reflected on their side and I should go to my bank and reverse the money. Because the money had already been debited I told the lady to leave it like that and just give me a confirmation letter that I have paid up the phone, she refused and kept on asking that I don't leave and take another phone🤷🏻♀️. After begging to given a confirmation letter which she still refused to give I was told to come back after the 1st of May. The only reason I requested the letter was that I don't want to be debited the next month since they have decided to take up all their money. A total of R7200 was debited instead of R498. I need this resolved and that they take their phone. I no longer want their services. It's my first taking up a cellphone contract and I'm truly not happy.