contract changed without permission

I have been with mtn for at least 10 years And this is the first time ive Had an issue. I got an sms last week informing me that my account was R2783 To be paid 3 nov. Which i dont understand because i have a my top up 100 account with them And have been paying R660 a month all along. So i phone the call centre saturday only to be told that my line has been opened and that the charges are for data usage. I inform the woman Jacquline it cant be cos ive never ope ed the line and my contract states its a mytop 100. She says she will lodge a complaint and it will take 3 to 5 working days give refs no. I phone back tonight only to be told that he cannot help me i must go to the store that helped me with my contract. I dont know what the hells going on but i can tell u one thing i will not pay for something i didn't not request and i will take the matter to the news paper if it is not dealt with. furthermore if it is not corrected and mtn take judgement i will go to court and prove that i didnt request the change and that i have a written contract stating what type of contract i have and i will win. to add insult i will hold Mtn liable for my costs incurred.