Dishonesty when upgrading contracts

Dear Telkom

I agreed to an upgrade over the phone with one of your sales agents a few weeks ago. The agent failed to tell me that he has removed the cap on data charges, as a result I have had to pay for data I had capped in the previous contract. Why is it my responsibility to make sure that the contract continues on the same terms and conditions as the previous contract. Your agent reassured me of this and then went around and did the complete opposite thing.