Kinda tough to replicate the service of a nation(global)-wide cellular comms network when that's what everybody on the other end of your calls and texts are using...

One option might be to use a non-cellular device (like an iPod Touch) connecting to a cellular hotspot with a "burner phone" sim (with a useable data plan)? "They" could still track that cellular device, but it'd up the ante quite a bit, since it would be very hard for an attacker to know the number. It wouldn't be "Screw you NSA, I've 'gone dark'!!!" secure, but it's probably stop a repo man or malicious ex from paying a bounty hunter to track you.

If you're prepared to drop the "calls" but, and fall back to just text messaging - have a look here: https://www.thethingsnetwork.org - there doesn't seem to be _quite_ enough coverage where I live to get reliable connectivity out of that assuming 2-3km LoRa range (although it seems like if I put one in at my place it'd fill in a coverage gap quite nicely...)

Apple Watch with LTE isn’t technically a cell phone, although you still need an iPhone if you go that route. You can just keep it plugged in at home all the time though.

On the ham radio side, Winlink 2000 is a mesh radio network that allows email transmission over radio frequencies. It runs parallel to the internet with nodes that interconnect the two networks. You do not need to be a ham radio operator to use the Winlink system - only if you intend to stand up a node with transmission capability. For more info see https://winlink.org

Assuming you're OK with moving to an internet connection for replies, FLEX pagers are a reasonable way to receive alerts. You can even do something nutty like put an SMS/MMS gateway on an android phone, with notification or encrypted messages delivered 'last mile' over the pager network.

I dream of a device with WiFi, LTE, and FLEX, with the ability to power on/off each subsystem individually.

Just get a faraday pouch/box for your phone? Turn it off, put it in the pouch/box, pull it out and power it on as needed.

Or just turn it off when not in use.

I see two options that address the concerns and goals realistically.

1. Put a dumb phone in a faraday bag for 99% of its life.

2. Keep a physical phone book and ask strangers or businesses to use their phones. Paying your way, of course.

What about a computer or tablet with a cellular modem on a cash-paid plan and a Skype account or similar service?

Maybe check how sailors do it when they’re in the middle of the ocean?

Expensive satellite communication networks.

For non-commercial use you and your friends can become ham radio operators.

I do have a license, actually. I’m not sure I can convince my relatives to join in on the plan, though. It would certainly work between friends with cheap Baofeng portable radios, though the range sure isn’t cross-country.

Edit: range wouldn’t be cross country on VHF or UHF, which is what Baofengs use.

You could build a relay system that lets you galavant across town while sending and receiving messages between your handheld radio <-> house/apartment <-> internet.

You can, but not in any secret way, at least not without breaking FCC rules. Encrypted comms on Amateur radio bands aren’t permitted.

Could get a satellite connection

it will be super expensive