MTN sets my usage limit without my knowledge or consent

I am having endless problems with MTN. This is by far the worst service provider that any one could ask for. I paid my MTN up account up to date up until my contract ends. So me thinking I'm doing something good then MTN does this to charge me even more without my consent. My account is paid up to date, i never missed one payment since I have been with MTN. I have been with MTN for more than 5 years. I paid my contract in advance because I have been having endless problems with them. So now to top it all off they send me an sms from them saying if I don't want my usage to be sit i must reply with No, which I did 3 times saying No, further in the sms it says if I don't reply alternatively they will change it to R738 on the 9th Dec 2019. SO I decided to call in only to find out that the "System" already changed it to R872.00 on the 27th November. I'm am so disgusted with MTN's service. I'm am going to take this much more further. Had I not called in I would've been billed almost double my monthly instalment which I did not agree to. I am going to warn everyone I know about the kind of things MTN does and will definitely make sure that the Ombudsman is aware of this. I have spoken to Mariam at 8:14am this morning. Ref 1-***