MTN *******ly increase contract amounts

I have had a contract with MTN for the last couple of months. Yesterday, all of a sudden and out of the blue, they informed me that my contract amount in increasing. How is this legal? I signed a contract with them for a fixed amount and package for a fixed period, how are they allowed to just increase the contract amount as they wish???? This is ********* on so many levels and *******! I should have never joined them! I want an answer from mtn, as this is not acceptable at all!