I recently "discovered" a super lightweight (2.2 ounces) waterproof cell phone, which is almost ideal for backpacking. I bought a refurbished Palm PVG100 on eBay for only $80, which looks and performs like new.
The specs on this phone are pretty impressive - it has an octo-core CPU with 3GB RAM, Android 8.1, and a 12MP camera which takes pretty good pictures. It runs all my normal apps flawlessly. Mine is unlocked and works fine on the AT&T network. Search YouTube for the trick to remove the SIM card.
The main disadvantage is the small battery. It runs down in about one day of standby if you leave cellular data and Wi-Fi and account sync enabled. With location enabled and a map app running, it loses 1% battery about once a minute! I normally disable all of these anyway, and I turn the phone completely off while in the backcountry. In areas with cell reception, I turn it on for a few minutes a day to text my family an update.
Of course the screen is very small (3.3" diagonal), and the virtual keyboard is tiny. It isn't an ideal all-purpose phone, but in a pinch you could use it for almost anything. I installed the "BIG Launcher" app which has huge fonts and symbols, which makes it usable without my glasses.