PROMO SERVICE FEE CHARGE

I renewed my contract a month ago May 2018 and opted for a sim only contract at R251,18.00 per month as I have chosen to keep using my existing handset. Since the upgrade I have been charged a monthly PROMO SERVICE FEE in the amount of R459.65 Excl. VAT. I have phoned the call center on numerous occasions, they unable to assist me as they have no knowledge of the contract change and are unable to tell me the new monthly charges and all that is said me, "Is Everything is correct and you are being charged correctly" . That is unacceptable MTN has no right to charge me and additional fee that is not stated on the subscriber agreement. I want this amount STOPPED AND REVERSED. How many other people is MTN doing this to?