bad communication from Telkom...

I took a fixed 24 months contract with Telkom which I pay 429 every month which I never had a problem until recently. I just received an sms saying I'm going to pay R618 for the month of April. When I called the call centre the agent told me an SMS was sent to.my phone about my spending increase which I never saw. I think it's unfair and highly unprofessional to assume that if I don't respond to the sms, I agree with the proposal.. please fix this query for me