MTN! REPAIR MY PHONE

I sent my phone for repairs through Mr Price, since I bought it there. A month later it came back and there was nothing done to it. They said said I should pay for the repairs because it's my fault the phone is broken. I mean the didn't last even three weeks. I'm still paying for it since I bought with the account. It's like they saying I should pay for the phone twice which is not fair. Please Help, what should I do.