No customer service at MTN

My contract with MTN expired, and I decided to switch to a cheaper one in September, when an agent phoned me. This contract was for R169 per month, and included a Huawei phone. The phone was delivered, but no contract. I did receive an SMS on 4 September, stating that I had been successfully migrated to the new contract. My old contract went off my mother's account, who died in February this year. The executors of her will tried to change the debit order to go off my account, and after months of back and forth, were finally told that I had to go into a branch myself to do it. When I went into the Rivonia branch, there was no record of me having migrated to an new contract. After a call to the agent in Durban, it was found that MTN was still debiting me over R680, now off a defunct account. This week, I got an email saying I owe over R680. When I phoned the call centre, I was told I owe over R1 000. And because I have no new contract, they are still wanting to charge me for the old one. This has been an exercise in frustration! Surely, it's not my fault if MTN doesn't supply me with a new contract? I am completely confused about what's going on, and MTN's agents have been frustratingly unhelpful.