Account problems 3 months not solved

Account was upgraded and the difference in monthly amount was suppose to be R20 extra p/m (middle October). Within 14 days I recieved my monthly account for November and I noticed a significant increase in monthly payment - from R1982 to R2914, with R20 increase new amount should have been just over R2000. I immediatly phoned Vodacom, and they informed me that they will correct the problem. I also asked them to reverse the transaction, since I do not want to pay more. Unfortunately I did not take down the reference. The incorrect amount was deducted. By end of November I received the new account, the amount was still incorrect and no credit stated. I phoned Vodacom a second time stating the problem (***), about the wrong amount being deducted. The person assisting me, saw the mistake and said that my account will be credited. I received month 3's invoice and this problem has still not been rectified. I would like this issue to be resolved immediately or I will need to move to another service provider.