No after customer care Bad Quality

I have upgrade my cell phone contract via MTN Bethal, received my phone in January 2015. Round about the beginning of February 2015 my Sony Experia Z3 back crack up completly, I took my phone in to Mtn Secunda the gentlement had a look at my phone and told me the following physical damage, The phone was in my handbag when I heard like a small explosion I mean really, I went on to Sonys web side and it seems to be a problem with this kind off phone now I must pay R2000 to have it repair, while it is suppose to be still under warranty. What do I need to do I'm only a student for teaching