Being overcharged on my cellphone account

I called Aufopage Cellular over a month ago to complain that I am being charged R323.50 per month for an MTN Top Up contract (line only) without a handset. This is ludicrous because it is a top up contract. I was promised that the matter would be rectified as I only pay R76.00 monthly for my other line which was taken out at the same time. Autopage owes me more than R1400 as I am in credit on my account, yet they are still trying to cheat me out of my money on a monthly basis by charging me R323.50 for a line which only costs R76.00