MTN LACK OF RESPONSE

I renewed my contract in June last year. I signed a new contract with a monthly payment of R99 and a once off extra charge for renewal for the first payment. I have been charges 33% more every month since June and cannot get MTN to correct the amount. I have been into head office 5 times and into the branches twice besides the countless calls I have made. I have taken a copy of the contract to MTN but every time I phone or go in, I get told they need a copy (which they actually have on the system under my cell number). I always get told they will 'ESCALATE THE PROBLEM' and all that happens is they FILE 13 it and never look at it again. I presume the definition of ESCALATION to MTN mean never look at it again as the client will eventually get tired of complaining. I have started taking down the staff members name and have a list of about 6 names. I cannot be the only person with this issue and would like to know from a Contract Lawyer that if MTN have reneged on the contract do I have the right to stop payment and go to another cell company? and how do I go about it then. If I take anymore time off work my Boss will fire me. Please can some one help.