Billing/ Data

I am one of the MTN Top Up client, in September I bought R30 airtime when I converted my airtime into Data it was not deducted (i used *141*2#) and I kept on purchasing data but my airtime remained the same and now I was charged on my account as if I'm on unlimited contract, now I got a bill for R4695.06 , my knowledge is that with top up contract you get a certain amount of airtime and data and when it's finished you buy your own and they don't charge you on your account, I have been trying to sort this out with MTN for 2 weeks and the people from their call centre don't send the requests or notes as per you explain to them. I have sent communication to retentions but the response is not up to standard instead I'm being asked what is the problem everytime via emails,I'm annoyed and I am not getting any help,I spoke to Masetshaba who's supposed to be a supervisor but she wrote the wrong notes when I phoned on Monday, today I spoke to Flora and she said When I was converting the airtime to data and saw that my airtime was not deducted but I was getting data I should have called MTN and asked why I'm getting data, who's problem is that if you have technical problems