Clear my name

I had a contract with MTN from 2017 until November 2020. I called MTN yo cancel my contract and the lady who was helping me confirmed that my contract was canceled. January 2021 MTN sent me an invoice ,which they said I owe them R255. I paid the money o the 25th of January 2021 since then I've been receiving invoices from MTN and when I contacted them they told me that I didn't cancel my contract and I kept on calling them emailing the proof of payment but until today I'm still stuck . Worse part they have handed my account to the depts collectors. Please help