How does a loose screen become my problem?

I'm not happy with MTN at all! I have to keep on phoning / emailing Mtn to follow up on the status of my phone that was sent in for repairs. When I phoned the MTN store in Three Rivers advised that it is beyond repairable. How did this became my problem? I pay monthly instalments for a phone of which a screen came loose. Why on earth will I make the screen to become loose. I would understand if the phone fell but this is not the case it just came loose. I managed for a while until I couldn't see anything anymore. It is also not the first time I'm complaining about the loose screen and every time MTN pulls up their shoulders. My phone is not even a year old it is still falling under the warranty either Mtn or Sony will have to look in to this matter as I am not happy with the decision made. Why do I need to be punished with a broken phone due to factory malfunction. I phoned the Sony call centre a few months ago advising them of the loose screen and they told me to go to my provider, which I did and every time there is a different story, I'm sure another service provider will buy my contract out moving my 3 contracts to a provider whom give high quality phones and helpful service.