being Held Liable

after upgrading my contract with MTN in November 2014 and taking an option where I explicitly asked to ensure that my contract continues like it did before which was on a top-up plan. needless to say this had not happened and during the month when I run out of DATA and I would purchase data as I should because that's how it worked before. in APRIL I am supp**** with an account that says I owe R3100 for out of bundle data used which I cant understand. so I go to MTN and they tell me its correct but no one can tell me why this happened when this should be a plan that needs to be topped up as I requested. then I was told I used a gig of data and hence the R3100 account because it is R1 per meg and yet a 3 gig data bundle is under R300. this is **************** and then when you try to have this reviewed I get told I am liable because the MTN \soft lock"did not kick in and like I say"