changed my phone to open line without agreement

I took a contract phone and I upgraded it last year it was on top up through out nd I told them that the new phone that i upgrade must also be on top up of which thy did but for two months my bill is increasing and i called them to enquire about the increase of the bill and they said my phone is now on open line of which i didnt agree with them to put my phone on open line someone did that without my agreement and they gave me numbers to dial and cancel the open line i dialed those numbers but again this month my bill is twice the amount that i have to pay and when i call them they say my phone is on open line still and that thy will be debiting money from me because i am on open line ,the problem is someone from mtn changed my phone from top up to open line without my agreement now they want me to pay for something that i did not agree on and they dont want to refund the extra charges that they are charging me