I'm not sure that you can launch the Camera app without first switching on the phone....by which I mean, waking it from sleep, rather than switching on from a fully powered-down state. I'm not familiar with your particular device, so for all I know it may be possible. With some Nokia phones, for example, it can be done by pressing the camera shutter button, and the LG G4 as another example allows it to be done by pressing Power and Volume Down together.
It may be worth posting in the Alcatel Idol 3 Series forum.
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