Vodacom contracts are a ****

My first contract with Vodacom ended in July 2016 and I started a new one with them in August 2016. I had a Uchoose Smart fixed contract with them but every month I would receive a bill with exceeding amounts. I paid the entire 24 months plus 2 months of a month to month when this contract ended in September 2018. In November, I called them to indicate that I would not be continuing with the month to month or institute an upgrade. My number was then moved to prepaid and I was asked to pay an amount of R436.01. I paid R450.00 just to make sure. Now Vodacom says I owe them R4000 for the handset since my contract came to an end. I do not know how is it possible to still owe a handset even after the contract has ended for 24 months. Every one I have spoken to from Vodacom says its an error from Cancellations department but I have not been able to speak to cancellations to fix this error because my number is now on prepaid. This amount is reflecting badly on my credit record and Vodacom does not seem to care for this, even though they have acknowledged the error. All standard contracts come to an end after the 24 month period and the customer can then decide to either upgrade and take a new device or terminate the contract and keep the device.