MTN- Only place to charge a promotional fee without even knowing what it is for

More than a week ago I've noticed on my invoice an additional charge called Promotional Service fee. I've contacted the near pathetic customer service line to query this fee. Approaching 2 weeks now and I'm still waiting for a response from MTN on what this fee is for, the person who assisted me during my call said, very unsure of himself, something about a promotional device fee. This is quite disappointing as my last device upgrade was almost 4 years ago. After explaining that to the telephone assistant he said he would raise a ticket and they'll get back to me as he was unsure why this service fee was charged. Guess what, if I haven't mention it before, I haven't heard anything from MTN, not even a ticket reference or any acknowledgement of any ticket that was logged.