Faulty Samsung Model SM-G313H

I have been a subscriber for a number of years and although the reception in my area is not great, I have never had these types of issues with any other phone before.<br> Six months ago I renewed my contact and received a Samsung Model SM-G313H cell phone - WHAT A DUD!<br> It reboots during calls, deletes my contact list.<br> I have been back to Festival Mall 3 times and every time when I have collected the phone I have been told \The phone is working 100%, there is no problem with the handset\". I am a pensioner