Cancelled Contract but I'm still being debited

My cellphone contract (24 months) with MTN came to an end in April and I called them to cancel the contract and migrate my number to prepaid but they never did that. They keep on debiting my bank account every month and I've reversed that last 2 debits. When I call the call centre they say it's been closed due to disruptions. Now they've disconnected my number. The ref # when I called to cancel the contract in April is ***6