Charging me for a product I canceled

I signed up for a Month to month internet package. Immediately I was double charged. I phoned the call center and they promised they would phone me back and explain it to me. As is always the case with mtn they never called back. I did however receive an sms saying my query is resolved. Out of frustration I cancelled the contract. Yet they still charge me for the cancelled contract. I've tried every channel and no luck