MTN breaching and changing cellphone contract without customer's consent

Reference number ********** 71. MTN Changed my cellphone contract without my consent or prior knowledge. I have a MyChoice 100 contract and realised with my latest debit order that my monthly payment was more than usual. I phoned them regarding this and they said that on the system they changed my usage limit although I have a top-up contract for the reason that I can control my spend. I asked the consultant (Zola) if she could confirm when and how it was changed. She said it was changed on 23 October 2018. I couldn't recall any interaction with MTN and would surely not have changed it as I signed that contract for a set monthly amount . I asked her to please tell me how it was changed as I did not give consent. She said that they sent a SMS to my number (which I did not receive, I checked) which said that I can get more minutes on my contract and that if I did not want it, I needed to send a word to "opt out". They didn't get a reply from me as I did not receive the message and therefore they went ahead and changed it. So she admitted that MTN changed my contract without my consent with an SMS. I asked her to change it back to R0.00 limit and asked for my money back as I did not give them permission to change my contract. The debit orders for January is already loaded and therefore I will need to be reimbursed for November, December and January. I am appalled at the ********** nature of their business and this is down right *****ing!! I will not let this go and will find a lawyer to take this matter further as I have a legal binding contract, which MTN breached and changed without my consent. This situation also has the potential to negatively affect my credit score as I am not aware of changes to the amounts of debit orders and therefore cannot be prepared for it. Something must be done and an example must be made of businesses that think they can get away with stunts like this and *****ing from their loyal, paying customers!!