Damaged Phone

I took a contract with Vodacom in July 2014. I booked the device in for repairs this August because it kept restarting and finally rejected the SIM card. I got a call from the repair centre informing me of the physical damage I caused to the phone. I then agreed for the phone to be repaired at my cost because I took responsibility for it. When I went to fetch it, they told me I had damaged the board (the motherboard). I refused to pay for it and I'm not paying. The repair centre has sent me pictures of the damaged board and not I'm even sure what I'm looking at. <br> <br> Please explain to me how I can damage the motherboard on a phone that I've never opened? And when you can't find an explanation, let me know why this is not covered by the warranty. This is bad! I'm not paying though, keep the phone. <br>