Most phones in the US “should” allow you but that’s carrier dependent. Much like getting your device unlocked to use with a different carrier.Most phones allow you to unlock the bootloader. Carrier provided Samsung phones in US seem to be an exception to this as the carriers lock down the bootloader unlock feature (but this should not be the case for most other brands)
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For battery life, this is actually preferrable. Basic black and white LCDs (what we would these days consider e-paper) are the most efficient (not counting e-ink screens, which are not available in this size I think)
I would expect very good battery life from this, especially considering you only need to use it for a few seconds at a time. And that's an important feature, because you're not likely to remember charging something like this regularly.
It's the same sort of display that was used in the original Pebble and I never had trouble seeing the screen on that.
I’ve had multiple times where the company swore up and down they unlocked the device for months and the device never got unlocked. So while it should be possible it’s not very easy or doable in most cases