Deliberate Horrible customer service

I app**** for a contract in October, which I cancelled a week later as I didn’t want it anymore. The phone arrived in January, after I had repeatedly called MTN and Mondo and I was advised that it had been cancelled. I returned the phone shortly after I had received it and contacted both MTN and Mondo but I received an invoice this week. I have emailed countless times and not a single person has contacted me. Instead I keep getting invoices and reminders of a debit order. I am beyond living at the incompetence and blatant ignoring of my emails from MTN. I need someone to call me ASAP and not to get debited because that will make me lose my mind completely!