Vodacom Can Alter a Fixed term Contract - As Per Help Desk

Got e mail to say that Vodacom was to increase my Fibre? Phoned *** this morning @ 08h09 to inform them that I have a "2 year Fixed Term Contract and they cannot increase my fibre! The response from the woman was that Vodacom can in T & C's. I informed her that this is not right, and is she saying that Vodacom can change a contract whenever they feel like it? I asked, "What do I do Now"? Response "Do Not Know"????. I asked again and she told me to go to "Retention Dept" I asked her "How am I to do this, why will you not help me? Please HELP!!!!