Mtn won't refund 18 months overcharging me.

For 18 months Mtn overcharged R59 per month above my contract for R160 per month signed November 2013. After 30 communications between me and customer care over last three months they will not accept liability. Only thing my account is now being debited with correct amount of R160 for last two months. I only use an iPhone and they were charging me for blackberry services which I definitely did not requst .. Reference number from Mtn is ***2. I have received ridiculous messages from Mtn since July 2015 whenI first reported the error made by Mtn in incorrectly debiting my bank account. As a pensioner I feel very strongly about treatment I have received.