Refusal to fix a phone that's under warranty and is a contract

Hi. I got an iPhone from vodacom(Phalaborwa) June 2024 via a contract. Somewhere April i took it back because it was not switching on then Vodacom (phalaborwa) took it to Polokwane repairs where they said it's the motherboard that's not functioning. The problem now is that the vodacom (phalaborwa) says they can't get it fixed because I don't have the purchase slip with me even though the phone is still getting paid for, under contract and under warranty. Now my worry is that the warranty is expiring soon and i still can't get help unless i pop out around R16000 to get the phone fixed which I don't have and don't understand. Please help