Faulty phone

I took a contract phone on some mtn shop it has been giving me problems after 5 months of using it. I took it back nd told them my story nd they told me to book for it so they wil tel me wat wrong with it nd i did so. After a week or so i got a massages from them telling me they are busy assessing my phone. 2 days passed by i got my quotation back they gave this huge amount of a bill that i have to pay for. They telling me my phone has software problem nd it a physical damage it switches on and off. I don't get dat how is that my problem because I ddnt even break it. I've been paying them nicely i ddnt even skip a month. Why are they giving us faulty phones and want us to pay for that. I am so disappointed that im even thinking of terminating this contract im not happy with this.