Contract cancellation

I've had a data contract with Mtn from 2019 December and it was ending 2021 December so I decided to cancel since-well the duration is finished. They cancelled it from contract to open line which I did not ask for, I’ve asked them to cancel to prepaid because I don’t need the contract anymore so instead they made it open line and they took their last debit in December month end … now yesterday they deducted 1500 from my account and when I call they told me I am on Mtn zero not contract but it’s still open …. Now when I call then they want to make it my fault and if I call them no one wants to help me .. they keep on transferring me to various managers … after all it’s not my fault, I can’t pay such money in January when it’s not my fault …. I can’t reverse my money because they’ll blacklist me over something I don’t know PLEASE HELP