Plan not explained properly

I took a contract with telkom about two weeks ago. When I took the contract I was under the impression that after the free minutes are over I would be able to buy airtime. I only realised after my minutes were finished that I was still able to make calls. I phoned customer care and only then I found out then that the plan I had taken required for me to call customer care for data and what ever else. I asked the consultant for a change in plan and she stated I must go to the telkom outlet and they would sort it out for me. I only got a chance to go to the outlet today and I was told I can't do the change. I am very unhappy about this and would either like to change my plan is take a higher plan then I have currently.