Mtn unable to bill correctly.

In April 2018 i took out a sim-only data contract for a double data deal. A very good deal 6gb anytime data and 6gb nighttime data for R149 per month. Fortunately for me I pay by EFT not via debit order because instead of R149 mtn decided to bill me R510. In May although i wondered why the bill was so high, but being ill wasn't able to do much about it so i paid. In June I started emailing 'customer services' with my complaints. At the end of June I paid what was actually owing which left a shortfall of R719.17. On 3rd July I was told accounts would have it all sorted out within 7 - 10 days. (7-10 days to put a correct billing code in????). Of course, nothing happened and on the 10th of July my phone was suspended and mtn started issuing threats of legal action against me. On the 18th July i received an sms saying my account would be activated within 2 to 24 hours. By the 21st it was still not activated and a visit to the branch got it sorted out. I was told all was well with my account. The billing i have just recieved is correct for the current month but the R719.17 has been left as owing in the 30 day column so i have now received an sms demanding i pay my current bill plus the additional money.

I do not intend to have this fight every month. I have just sent an email in which i have said a) if my account isn't corrected by 31st July I will be taking legal action against mtn and b) if my phone is suspended i will regard it as MTN cancelling my contracts and will pay no future invoices.

I can't believe a company as large as mtn can be so ***********. Nor can i believe any company can treat a customer of 21 years so poorly.

What else can i do?