Vodacom has been robbing me for the past three months

Vodacom has been robbing me of R47.83 for the past two months. I have a contract with them and they claim that I took an advance in March, which I subsequently paid for. The same charge came in my April, May and June statements and they claim that I have not paid these. I have called the call centre and a dispute was lodged. I sent a screenshot stipulating that I do not have a balance on my airtime advance, nobody responded. I got my June invoice and I raised the same issue again, nobody got back to me. Is Vodacom in the business of robbing its clients now, and ignoring them when they query the issue? I am really unhappy with the service!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!