Have to pay R35 for defective Sim card on contract

My wife's phone recently started having a network problem and is constantly saying insert sim card. We went to a Telkom mobile store where we were told the old 8.ta sim card is the problem. We need to do a sim swop @ R35. Understand it's not about the R35 but the principle of it. The Sim card has never been removed from the phone since it was taken, so it's not physically damaged. Telkom upgraded their 8.ta sim cards for a reason and this is one of them. Why must we pay for a Sim swop when in fact we don't want a sim swop we just want them to fix the phone as it's under warranty. How is it the customers fault that the Sim is defective? When we asked to book the phone in as we were not looking for a sim swop we were told we must bring a copy of the contract before they can book the phone in, and if we lost it we must go back to the store where we got the contract, never mind if that was in another province. Funny enough Telkom gives free sim cards and sim swops for prepaid customers. Anyway, we don't want a sim swop, fix our signal/insert sim card problem. If a sim swop is what it takes to do that, YOU pay for it, not the already paying customer.