contract breaking

I have a Vodacom contract on a Tabloit.<br> This contract was for 2 years.<br> Recently I was informed by vodacom of a change in my contract by means of a change in monthly tariff.<br> I sent a email to vodacom and they rep**** that a consultant will contact me in 48 hours.<br> That never happened.<br> So at the end of this contract at month of June, I am also ending any further deals with vodacom.<br> And even now, I am on MTN prepaid due to vodacom's bad service.<br> So vodacom can change a contract as they want ??