Poor Service and Overbilling On a Non Existent Contract

I've spent the last two months attempting to get in touch with Vodacom about my contract issues. Despite my contract's conclusion, I'm still receiving bills from Vodacom. Originally, there was a cap on my contract, but now it seems to have been changed to an open-ended arrangement, with potential charges reaching up to R2000. Each time I call Vodacom for a resolution, I end up interacting with their less-than-helpful AI system, which hasn't been able to assist me at all