Breach of contract

My name is Bonnie, I signed a 24-month contract with Vodacom with a monthly premium of R249 only for a cellphone, Huawei P9lite. I was happy with the contract until Vodacom started deducting more than what we agreed on, in a contract that I signed. They started by deducting more than R250 from there, they deducted more than R298.

How can I sign a 24-month contract with a premium of R249 then the service provider just violate it by increasing the monthly premium on that contract? My account number: QC294963-8.