Phone Damaged after sent in for repairs

I purchased a Samsung Galaxy S5 from Cell C on the 22nd of March 2015. I used the phone for about 3 weeks when it started freezing and the screen didn't' work. I took it to Samsung Mall of the North where they did a software upgrade. But it still did the same.By the 11th of May 2015 I booked it in for repairs at Cell C Mall of the north. They phoned me yesterday and told me my phone is back from the repair centre. When I saw my phone I thought it was someone else's. It's damaged on the sides and the report said that they had to replace the screen. I have proof that my phone did not have a mark on it when it went in for repairs. I would also like to know why they are not replacing that phone. It wasn't customer negligence. It was a hardware problem. It's unacceptable to have such an expensive phone and it only works for 3 weeks. I can't complain of the service I got from Cell C mall of the North. They still assisted me with a smile. But I am really disgusted with the repair centre'.