MTN call centre not very effective

MTN have still not passed a credit on my account due to them not having upgraded my phone contract to the correct contract after they failed to update their system, and by doing this charged me incorrectly for data. I then call them on many occasions and after a year have still not been able o talk to a person that knows what they are doing. I have been told a few times that a credit will be passed, but nothing as yet, I then call again and get told that there is no credit now. Surely MTN has some skilled people there that can sort out a small credit issue, I have even done a break down for them for ease of reference so that they can see where the credit comes from but I don't have any details of a person to send to because the call centre is utterly useless!