Lenovo — my phone switches off after my completion of my phone call.

After the rejection of my phone call the phone switches off every time. A and i also contacted to the service centre but they also failed to repair my phone. My phone is still in a warranty and sometimes the touchpad is also turn in to a dead condition. And it always works after the reboot of the phone. It is very annoying. And i'm suffering from this problem since i got this phone. So i'm grabbing your attention towards my phone. So please help me...