Pathetic service

I closed my account in 2019 (liaised with retentions, paid the money for earlier closure/cancel contract) as I immigrated. I AM NOT USING THE NUMBER SINCE 2019! So this meant no debit orders as my contract was paid up and cancelled. HOWEVER, someow MTN decided to decided to deduct money from my bank account end August 2020 (1 YEAR LATER) because my contract is now going over to a month to month (some call centre agent told me). How the hell, it was cancelled - but clearly this was beyond his knowledge and skill to understand. still 3 weeks later, nor retentions, nor customer care, nor MTN came back to me, or to refund my money they wrongfully decucted from my account to which they had NO authorisation to do so! I demand a response into this issue!