Contract changed without my consent

I took a month to month data contract in January for R99 and in April I recived a bill of R306 and I was told it was because I went over my limit which was a bit strange and I then wanted to cancel It. After weeks of trying to cancel , I wa la told that my contract was migrated to a 24months fixed contract in Feb after they apparently sent an sms to my data simcard which stays in my router 24/7. I then asked why wasn’t I given a telephone call on my normal line as per usual . I wasn’t given an answer . The migration was done without my consent and I would like cell c to explain how do they migrate a new customer without their approval. That in my books is *****