Terrible service

at the end of October MTN credited my account, when I queried why they said it was a system error and they calculating my account wrong in the pervious months, the guy said at the end of November i will be credited as well. I asked him to be sure because I didn't want to be charged double and he assured be it was correct, on my November statement it showed that MTN owed me R65 and my account was not debited because of the credit they owed me covered my subscription for that month, However in December statement they charged me an extra R489 because they said I didn't pay my account in November because they had a system error and I should not of been credited, my Bill for December comes to R1202, I keep querying as tp why This is happening and no one can answer me, it is impossible to speak to a supervisor to find out information, I have been a loyal customer of MTN for Over 10 years, I want to know from MTN what is going on and how long are they going to over charge me.