MTN contract cancellation NIGHTMARE!

My mother and I have been trying to cancel the contract we have with MTN for the better part of 2 years. I hadn't upgraded the phone and wanted to switch to prepaid as she'd been paying for it. MTN told us we can only cancel in the month that the contract was taken out. We waited until then; that's when the **** really started trickling down. Their contract department number was constantly unreachable, the few times that they were they'd send us to 135. On 135 we were told they don't deal with contract numbers, we should call 083108, then 808, then 135 again. Today they locked by sim because they didn't debit one month back last year! That amount has been rolling over from month to month but we keep getting sent back and forth when we try to find the people who can help settle this. It's been ****ty to say the ****ing least.