MTN OPEN ACCOUNT VIA SMS

Apparently MTN sent out SMS' in August 2019 to their clients proposing to open their capped accounts and bill them for going over the limit, if the customer is not interested they need to reply to the sms with a "NO". I have always capped all 4 my numbers with MTN and don't have any record of receiving this sms! I have now been billed R2000, because apparently I didn't reply to this so called SMS with a "No". If MTN wants to propose something like this it should not be sms'd but an agent needs to call me and propose it, like with any other contract that needs to be agreed apon by the client and not via SMS which I have no recollection of!!! I have now contacted MTN and they are not able to provide me with proof of this SMS. Whether they can provide proof or not, this is still not how you inform a customer that their account is suddenly an Open account! I need to be refunded asap!!