Breaching of Contract

After endless problems with invoices issued for my account, I have to hear that my contract fee is not a fixed amount of R599 what I signed up for but R608.50. How can my total amount change without my consent or knowledge. A contract is signed for a reason and now it is being breached? I don't care how little the amount is, that is my money that Vodacom wants to take without any approval or consent! My colleague has the exact same contract as me and she is just being charged R599. Why am I the exception and why does my contract that I signed not apply to me?!