Impossible to cancel contract

I have been trying to cancel my contract with MTN for over a month now. My contract lapsed and I wish to move over to prepaid. I have been calling 808 and they keep telling me that someone will come back to me. I called on the day they specified and also visited the store, where they keep telling me to call 808. I have already called and all they give me is a reference and say that someone will call me back. I am still waiting and it has been more than a month now. This is unexeptable and I would appreciate assistance with this.