Overcharged by MTN

I renewed my contract in February on a corporate package which entitles me to a small discount, MTN has been charging me the standard rate, I raised this with the outlet where I renewed my contract and they asked me to bring a copy of my Salary slip which I did a few months ago, this was not sorted. <br> I then called the call center on 6 November, they told to to email my salary slip to them which I did, still no resolution. I escalated the query on 17 November and I still haven't gotten a response. <br> <br> Very disapointed at MTN for *****ing my money after been a loyal customer for over 16 yera