Q: iPhone always restat itself

Howdy there bw31,

If I understand correctly your iPhone 6 plus is just restarting on its own, whether or not you are actively using it. I would start by resetting the device:

Restart or reset your iPhone, iPad, or iPod touch

 

• To reset, press and hold both the Sleep/Wake and Home buttons for at least 10 seconds, until you see the Apple logo.

If the issue persists, backup and restore your phone as new with this article:

How to erase your iOS device and then set it up as a new device or restore it from backups

If that does not resolve it, then use this article to udpate your apps, and device or reach out to support if needed:

If your iOS device restarts, displays the Apple logo, or powers off while you're using it

Thank you for using Apple Support Communities.

All the best,

Sterling