********** billing of new Mtn contract!!

I upgraded my mtn contract in 13 July 2015 to a cheaper contract of R279pm to receive R110 airtime a month. I agreed to this contract because I was informed that it would work as a topup just like my old contract and that as soon as the R110 is completed, I can buy prepaid airtime. Yesterday (23 July) I get an Mtn statement for about R800 for the period 20 June-20 July!! How the hell is this possible when my old contract was the 200 topup, I have been on my new upgrade a week and I asked for a cap once I finished my R110 so I could know to start using prepaid?? I spent all of last night and this morning calling the mtn call centre and keep getting disconnected!! I am beyond angry now! How can I be billed like this when I purposefully wanted a cheaper contract and asked the right questions and explained myself to make sure this exact thing wouldnt happen?? Someone needs to get back to me asap!! Mtn you have become a useless service and Im stuck with you for 2years now!!!