Incorrectly billed and told to pay!

I have been with MTN for more than 10 years. last year i took out a closed line contract (top up). it was explained that when the air time is finished, we would need to physically load more before we can carry on making calls et etc. this has been fine for over a year. all of sudden we have been hit with a R2000 bill. MTN has claimed that it is correct because we are using packet switch data which means when you airtime is finish, it switches to this mode and bills you at the end of the month. we have been having endless fights for past 3 1/2 weeks in trying to fix this incorrect billing. they now claim we only changed the limit on 13 October 2015. How do you change a limit for R0.00 to R0.00 which is the sms we received. A supervisor at the MTN customer care even checked the account and admitted that we are being billed incorrectly and she would have it sorted by today 29/10/2015. our line has been cut. I call in and get told they don't know who helped me and they can help me with a payment plan. I