MTN Bill

I currently have a MTN contract of R99 a month. On the 17 January 2019 I received my statement of R598.99 I called in on the 28 January 2019 to query it as this was way over the amount that im supposed to pay. The lady explained to me what has happened and that I used internet or calls out of data hence my bill being that amount. I understood that and asked her to change it so that it does not go over the original amount of R99 that I am supposed to pay every month, She said ok it was change on her side and gave me a reference number. Today I open my email to see that MTN billed me again R598.99 for the month of February, the exact same amount that I was bill last month (January) after I called in and asked then to change it so that my bill dont run up so high ever again. I called in as I was furious this time around, the lady checked and said to me the lady I spoke to prior made a note but it was never changed? Hence my bill being so high again this month. I have been using MTN for a very long time now and are disgusted with the service I received. So who should be held responsible for this bill now????