MTN reaping me off

I took out a contract with MTN and it ended February 2022. I called in April to give the 30 days notice of cancellation. They said the cancellation will go through in May. I then received the May bill and I was confused because my contract is supposed to be canceled. When I called I was told there was a system error and it canceled late. June I received another bill. Now it’s July and I just received yet another bill. When I call I am sent from department to department with no assistance. They are messing up my credit report because they put me on “default”. They told me the cancellation went through in May but I still receive invoices. Why am I supposed to continue paying for a non existing contract??? I called more than 5 times only to be told different stories. I even tried them on Twitter and they said they escalated the matter until I received another invoice… what do I do?