Cancelation of upgrade, rectifying of monthly bill.

I called MTN on the 18th of August 2020 to arrange for the return of the sealed device as I cancelled the upgrade. I was transferred to an external service provider where I was informed that my entire contract will be cancelled and my number will become a prepaid number if I returned the device (even though it's sealed). I did not want my contract to be cancelled as I've been a contract client of MTN for many years, I just want to return the device and then upgrade when I'm ready to do so. The lady on the line arranged for a courier to collect the device as there is no MTN store where I can return the device to. I've asked for a reference number and she was unable to do so as my contract number is the reference number. I've canceled but still being billed for a device that was returned to Mtn on the 24th of August 2020. I have photos of sealed device that was never used and not even opened along with the collection notice. The upgrade still have not been reversed and Ive been billed for the device since August, now every time I try to make a call, I cant as its saying I need to load airtime, where I never had this issue before. Kindly rectify according as I've been trying to reverse the upgrade since August, its now 27 October and its still not resolved. I've send various communication but have not received any response except the monthly incorrect bill. I do not have the said device and the matter still have not been resolved and I'm still being billed for a device that I don't have. Im quite shocked as I never in all these years had any problems with MTN.