Billed incorrectly once again - Extremely upset!

This is now the second month of me being billed incorrectly by Cell C, I received a call from Nelly @ Cell C after escalating this issue for the first time who has been assisting me with having the bill corrected for October and the correct amount debited and billed correctly going forward. I took out a data contract for R399 a month which is a special they had on, on the Huawei E5577 LTE Mifi Router. Nelly had called on Tuesday 1st of December to advised that she received feedback stating that the billing issue has now been resolved and that I will be billed correctly going forward. I then received another call from Nelly on Tuesday 8th December advising me that she is still waiting for the new itemized billing to see that is has been resolved. I have just checked my statement now online for the month of November and Cell C once again billed me R950.13 which is totally incorrect, this amount should be R399. I am really and extremely peeved off with Cell C and their systems, they are quick to get customers in but not to resolve issues once and for all. Their accounts division is pathetic and useless!!! I need this rectified once and for all as I cannot keep going through this!!!