complaints about complaints

Its now been 1 year and 5 months since I return to MTN what belongs to them. I've been handed over to their useless debt collectors and have phoned countless time to find out how far this cancellation is. This has been escalated every time, after the 2nd escalation I was told that I have to phone mondo to cancel the contract (which I have done) but according to those brilliant service champions at MTN they do not accept Mondo reference. Everyone and their aunt has the return slip from DSV, their are communications where Mondo confirms that the device has been collected. But for some or other reason I still have a default on my name. Okay so the last time I phone the **** center I was told to raise the issue with their so called complaints department. It has been three weeks and no one has responded to this. Now I'm not asking for MTN to respond but I need more evidence for the case I'm building