My iPhone 6s keeps shutting down when I perform two actions

My iPhone 6s keeps shutting down when I perform two actions:

1) When I try to make or receive a phone call, within 5 seconds, my phone shuts down and reboots.

- A small workaround solution that I have found for this, is to make phone calls via my bluetooth headset or my standard apple headphones. I can make and answers calls without any problems when I use this workaround solution. But if I answer the phone without a headset, the phone shuts down and reboots.

- One other small note, before my phone started to have this problem, I noticed that the proximity sensor was not recognizing when I put the phone up against my face. The screen would remain on.

2) When I try to use the camera my phone shuts down and reboots.

The phone shutdown and reboots every time I perform these actions. It doesn't matter if my phone is at 100% or 1%.

This is very frustrating! Any ideas or things I can try?

Repair options that I've seen on different forums that could be a potential solutions: replacing the battery and/or screen.

Things I have tried to resolve the issue:

- Soft reset

- Reset all settings

- Reset all content and settings

- Reset network settings

- I've removed and reinserted my sim card

- I've removed certain apps that I thought could be causing the problem

***I bought this phone used and it worked flawlessly for the first two months. I live in Guatemala and am using a local network here (Tigo). I first used it in the States over the holidays on the LTE network by H20 Wireless. Then I changed it over to the Tigo LTE network here in Guatemala. I don't think this info matters, but thought I would include it.

Thanks for any helpful info you can give me!!!