mtn cheating me /

this is my second complaint, i have three contracts with mtn , the main sony z3 for R499PM + SAMSUNG NOTE 10.2 R 389pm and samsung s4 mini for r224pm/ = my total bill should be +- R 1200 p/m and all contracts as agreed should be on a top up basis, meaning mtn give me free data and minutes that is limited, once that is depleted i will have to physically top up my air time, that is the way it has been for the past 2 years, when i upgraded to the sony z3 in January at mtn the glen branch, we agreed that the in-contract would remain exactly the same, only difference is that the z3 would cost R 499 a month , instead of the R 399 that i paid for the previous phone, every month since then, mtn has been cutting my line saying that i need to increase my limits, when calling the center , they cut the calls. i spoke to carmen from the call center who now advised me that mtn is charging me an extra R 914 for internet excess usage, when advising her that its impossible, she advised that there is nothing she can do until i settle that amount. so i want to know why is mtn putting billing charges on a top up contract, that we can never use in peace with being cut off in the middle of the month ?