Useless MTN

I have had a contract with MTN almost 4 years, suddenly in the last 3 months they start charging me a different amount to what I have been paying all along. I have called them multiple times and they advise that there is a promotion fee on my billing which never use to be there. The promotion fee is apparently an indication of a device been added to the contract. I am almost done with this contract so why would I add a device 18 months into the contract?, I have asked them to remove it because it was not something I agreed too. I always get told "We have logged an investigation into the issue" then nothing happens and debit order day comes along and the wrong amount keeps being taken. I am tired of spending hours on the phone with MTN trying to get this resolved and nothing moves forward. For a brand like MTN to provide such poor service is unacceptable. Can we not do our jobs?