Unauthorised changes to contract

My problem started 3 months ago when I realized that I can't send sms's any more. I had numerous interactions with customer service who reset my message centre number etc without success. In December a store assistant helped me and discovered that I don't have an sms bundle linked to my contract. I'm on a a MTN choice 200 contract which comes standard with 300 sms's. In the middle of my contract for no reason at all my sms bundle was removed form my package by MTN. This is a serious breach of contract since I have been paying and continue to pay for this contract and service. I am considering cancelling my contract which I am fully entitled to do seeing that MTN are in breach of the contract I have with them.