Increased my debit order without my consent.

I signed up for a contract for the amount of R150 per month which was debited for at least 1 year. Without my knowledge, Telkom has increased my monthly payment to R250 , without any added data and if there is, i do not want it. I have logged calls at least twice, only for them to come back and tell me that i am owed a refund by them. That is not the point! I have received next month's invoice and it still has the wrong amount. This is theft!