I have a contract with Mtn, it's a Sim only data contract

I have a contract with Mtn, it's a Sim only data contract. My ex husband has been able to call in and make changes on my contract because he knows all the answers to the security questions. When I tried to cancel the contract that got stuff in the process. Calling 135 is pointless as the call either doesn't go through because they are offline or it get dropped midway. I have been to multiple mtn outlets to try and resolve my issues and they all tell me different stories. I can't change my number because of the cancelation order and I can't cancel the cancelation because that should only be done via 135 which is not working half the time.