I'm being over-charged

For the last 2 years I was with Nashua Mobile(NM) and I never had any problems with my contract. And when NM closed down my contract was automatically transfered to MTN, that is when I started having problems, instead of paying R199 a month I was being charged R260. I phoned them and they promised to reverse the money, of which they never did. In March I renewed my contract, the package I took is for R199 and yet again I'm being over charged, last month my account was suspended, and when I phoned the call centre, I was told I exceeded my limit. And my confusion was : what limit I exceeded? Cos the package comes with BIS(so it can't be the internet, and I don't even download staff) and I hardly make any calls using this phone! Right now I have been holding the line (808) for 2 hours cos I'm trying to get some clarity, as usual the call centre is forever busy!