Over charged

When I took out my mobile contract with Telkom I requested for a limit on my account. This contract was for my mother. When she travelled overseas she made calls thinking it's her free minutes, however she was charged for these calls and the limit never kicked in. When I called the Telkom call centre to query the bill the agent said that some other limit was not activated and I had to request it. I told him that how am I supposed to know Telkom systems - I asked for a limit to be set and Telkom being the experts needed to do so. I asked for these charges to be reversed seeing that it is a Telkom fault and was told no it cannot be reversed and I am liable. My contract is coming to an end soon and I will be porting to Vodacom after having this contract for many years!