MTN is the most unhelpful Corporate giant I have ever dealt with

I returned a cellphone in July 2020 as MTN had upgraded the wrong contract. They have continued to debit my account every month, and have now tried to debit the full amount as a once off debit for nearly R20,000. I have sent numerous emails, messages and made numerous calls to them and nobody gets back to me. In future, I will be moving to another service provider and will discourage anyone from taking a contract with MTN. This is after having my contract with them for around 25 years