Charging without my knowledge

My sister bought me a phone on contract which initially coated her R320 ,the contract was fine till it was my 12month using the phone .Telkom sent me a text saying I qualify to extend my airtime to R500 and if I'm interested I should reply with a Yes.I didn't reply next thing month end of November 2015 I was charged +-600 ,I called in at the call center I explained my situation after the agent told me their systems are down ,I tried again I was told again I can't be helped .eventually this month I called again the systems where working I explained this agent told me that by me not replying it means I agree with my airtime being extended I asked how is it that telkom can take a decision without my acknowledgement to me this is *****.I asked to speak to the manager she said she will escalate the problem to accounts an billing department to date I haven't received a reply or anything .the account is under my sister Tsholofelo Malapi ID Number ***