MTN not returning my credit

I cancelled an MTN contract within 7 days of receipt of the device- I returned the . device within 7 days and they confirmed that it was in its original condition

When I returned the phone I was warned that there is a backlog of returns so it could take a while for the cancellation to be completed, i.e. I may still be billed until the cancellation is effected.

I have been billed- for months. When I called in recently, first week of Jan, I was assured that the cancellation has been effected. Indeed, I have not been billed this month (or maybe not yet- I hope not).

I was promised that the credit due to me would be finalized in a week...it's been 2 weeks and counting...I have called to follow up and told the same thing.