Top - Up Contract Limit not

I recently renewed my data contract with Cell C. All communication was done via the phone. On the phone call I requested that my contract be changed to a Top - Up contract so that we can no longer go over our contract limit without purchasing further data bundle. This month I checked my data usage and it is already way over our contract limit. This is not what I agreed to when taking out the contract and understood that i would not be able to go over my Top-Up plan limit.