Vodacom is harassing me

Towards the end of November 2018 I purchased a Samsung A6 + cellphone via Vodacom contract since they were running black friday specials and 5 days later I returned the Samsung A6+ within the 14 day cooling off period made the call on the 11th of December still sealed and spoke to a guy called Godfrey Khumalo who assured me that this cancellation will be processed.In January I received a statement cellphone saying I owe vodacom for the A6+ and the money was deducted from my account which I later reversed from the bank. When I received the statement I queried with Vodacom only to find out that the cancellation was never processed. I have been sent from pillar to post without any assistance ever since. They gave me a reference number: ********** ********** 9. I'm being threatened that If i don't pay this amount my other contract I have with them will be suspended until I settle. My question is how can I pay for a device that I had returned and never ever used. Please assist in resolving this problem