Poor service and poor communication and no accountability

I had 2 home Wifi's with MTN the contact expired and i went on month to month for about 4 months. I then upgraded so i went from paying R1400 to R799 per month. 1st month i paid Pro rata R1200 and 2nd month R799. by the 2nd month i received a bill for R10k why? i simply upgraded. I called the call centre they said its an error on their side they will fix it. Then i assumed all is in order. Then the next month MTN called again saying im overdue R10k i told them i am in no arrears i have been paying. This has been going on for 6 months now. Then on Friday without my comms my services was suspended. I called the call centre and they said there is an error i need to speak to the accounts department and they transferred me knowing that department is closed as i called 17:10. Now today i get a sms survey for a cancellation? This is bad from you MTN an error on your side affecting me for 6 months on my credit profile and each month i have to tell you the same things fix this please.