I should not pay for MTN's mistakes

Last month I received my monthly invoice from MTN. It showed that there is a credit note for the exact amount of my monthly installment, thus it showed that I owed R0.00. I then assumed that I paid more than my normal monthly installment over the time and accumulated as credit. This month when I received my invoice on 14.05.2020, it showed that I am over due with last month's installment. On 18.05.2020 at 22:00 I received a SMS that says "your order has been submitted". After I received this SMS, all my phone services were cut off. That same night I sent MTN an email asking them to rectify their mistake. I certainly should not pay for their mistakes. Up to date, I have not received any feedback from them. I am running my own business and since my phone services are cut off, I am not able to receive business calls. So who is going to be held liable for the business and money that I am losing?