No accountability - Billing Issue

I had a contract that was running to an end in December 2019. Knowing that Vodacom does not cancel contracts until you call in to cancel it, I called in to have my contract cancelled. Since there was a confusion on Vodacom's side around when my contract was supposed to end, the representative I was dealing with said they would log a query and come back to me, which they did and advised that my contract would end in December. Vodacom continued to bill me for January, February and March 2020. I have called in multiple times and have spoken to multiple representatives who have promised to investigate and give me feedback, to which none of them came back to me. I visited Vodacom's head office in July and was advised that they cannot help me as my account had been handed over to legal, I was given an email address but no one came back to me. Lo and behold, I have just found out that they have issued a handed over default against my credit profile. I am utterly disgusted about the lack of ownership and unwillingness to help that Vodacom and their representatives have shown. As I have stated to almost all their representatives, I don't have a problem paying the account but I am disputing the additional months I was billed for while having cancelled my contract and not using any of those services I am being billed for.