Unsatisfied vodacom contract customer

I have recently upgraded my contract with vodacom and my handset is still under warranty i had a problem with not hearing in a telephone conversation, so I booked the handset in for repairs upon assessment the vodacom representative tells me the warranty is voided and I will have to pay cash because I had insert an another sim card in to the device. I don't understand the warranty should be with the manufacturer and not service providers, this totally unacceptable. Vodacom is ripping their customers off and I am really not happy about this..