MYN cancellation of a contract problems

I was contacted by MTN in December 2018 and agreed to purchase a 'special' deal from MTN over the telephone. I thought it was a great deal as it included two ZTE Blade A110 cellphones for the price of one . Big mistake . I enquired from a privateer and was told that the phones were allegedly 'not as good' as others available . When I wanted to cancel the deal I was told I would have to pay a hefty penalty and should go ahead and use the phones- I could only cancel the contract in December 2020. So I bit the bullet and tried to use the phones ,but could not transfer any of my data from another phone to one of them so I simply stored them in a drawer , till October 2020 when I upgraded my own personal phone with MTN as it was due for an upgrade . That went well and I was happy . I asked the MTN agent at Metlife Plaza MTN in Port Elizabeth when exactly I could cancel the 'special deal" I had contracted in 2018 and was told "anytime between 23rd November and 23rd December 2020" (quote). So I contacted a Sindele on 083135 at 15h10 and asked her to please cancel the contract.She said okay and gave me a ref.no. ***. On 21st December 2020 I again phoned 083135 at 11h20 and asked a Thabang to verify that contract would be terminated on 23rd December2020. He could find nothing to indicate a pending cancellation,so re-did the cancellation and gave me ref. no. ***. On 23rd Dec I again phoned to confirm the cancellation and was told it had been cancelled, BUT I would have to pay the invoice which they had processed on 17th December 2020. I asked why , as the contract has been cancelled, and why invoice me for a service which was to be cancelled six days later? I was told that my contract was always paid in ARREARS . This is of course nonsense and I said so . So what does one do in a case like this? If I do not pay , will I be black-listed or my present MTN phone be terminated? Has anyone else experienced similar problems in trying to cancel a contract ? Do I need to contact the ombudsman for cellphones in South Africa?