Phone switches itself off

We have returned to the first device, because it has always turned off independently and there is no way the unit for a different setting. By the manufacturer, there has been no response to requests; only the money has been refunded back immediately. Ordered again the phone, the same error again and no response from the Herstellers.Spätestens after 3 hours, the device switches off completely (not sleep); ie, it is no longer accessible. We will also send the second device back and opt for another model. Too bad, because the device has liked to be and was well thought out.