Dead Cellphone / Handset

In June this year (2015), I upgraded my handset and requested for Sony Xperia Z3. I was given the phone and a day later I returned the charger as it was not functional. A week later I was given another charger which also did not work. I then decided to drop the whole thing and use my old Sony charger as it had no problem. My new phone started giving me problems around October 2015 where it will get very hot, freeze and switch off. it will then take a day before it restarts again. I sent my phone to Cellucity (Amanzimtoti) and it was booked in. A week later I was called to collect a phone and I was told it has been repaired. The very next day the phone heated up, froze and switched off. I took it back to Cellucity and again it was booked in. On Wednesday the 2nd Dec 2015 I was called to collect the phone and on the 3rd Dec 2015 the phone heated up, froze and switched off. When it switch on this morning none of the applications were working not even the SMS could work. I am tired of this back and forth, I need this resolved ASAP. If they cant fix the phone, i need a new phone as it still under warranty. I have lost so many clients due to my phone not working. Please help me.