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Well due to the fact that you say it shuts off approx 20 seconds after initial power-up, I'd suspect a battery problem. Does the phone power completely off or does the LCD just turn off? (Verify by trying switching phone into vibrate-mode with the slider button and see if phone vibrates).
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It does not vibrate it all. I checked it. thanks
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Ok. If the phone is brand new I would recommend bringing it to Apple for investigation prior to mucking around with things and voiding your Applecare (if you have it).
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One thing you COULD attempt without any problems is perform a soft-reset if you have not done so already. To perform a soft-reset, do the following:<br />
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When the phone is on (during the 20 seconds...) hold down the HOME button and POWER button <b><u>together</u></b> until the phone restarts. Then see if the symptoms are the same.
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we tried that few times. thanks
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The phone is brand new, but when I turn it on after 20 seconds the screen blacks out and then turn off the phone. I am not sure why its doing it. Please help?
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The phone is brand new, but when I turn it on after 20 seconds the screen blacks out and then turn off the phone. I am not sure why its doing it. Please help?

Iphone 6S IOS 9.2

The iPhone is a smartphone made by Apple Inc. It and the iPad, along with several other mobile devices from Apple, run on the iOS operating system, which has its own topic, as does the iPad for device-specific questions. Native iOS applications are written in Objective-C or in Swift using the Cocoa Touch frameworks or HTML5 compiled with the help of a package manager such as Adobe PhoneGap; a common IDE is Xcode.