Why does my iPhone keep resetting?

I have an iPhone 6s Plus, and I believe the IOS I have is an older one but I'm not sure which one. It's been resetting lately like my phone will just stop playing music and functioning then the screen turns black and it does the white ring thing that usually means something is loading, and then it puts me on my lock screen and I have to put my password or finger print in. Why does it keep doing this? Is it cause my phone isn't updated? Once when this happened all my pictures and text were gone but the pictures ended up coming back but not my messages. Does it have to do with the fact that I have no storage on my phone left? It's just annoying I want to solve this problem. I've asked ppl and they say it's happening to them too. Someone tell me what's wrong please and thank you!

Updates

Why am I being notified that a call may fo to "3others"

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