Travel backwards in time. Get the oldest smartphone you can. Then next year, get an even older smartphone, then go back to a flip-phone, then regress into a candy-bar phone, every time a device manufactured earlier and earlier, with fewer features, fewer chips, fewer peripherals (cameras and mics), less bandwidth. Removable battery. Memory just for the numbers of your contacts.
Unfortunately, that doesn't work, because the old cell network radios don't exist anymore.
You can get a modern flip phone or such, that supports things like VoLTE (AT&T is dropping support for all non-VoLTE phones here... soon, if they haven't already), and there are some candybars, but going with "older devices" will soon enough prevent you from connecting to the cell networks at all. 2G is gone in most areas, 3G is going away, etc.
About ".....old cell network radios don't exist anymore...."
I've often wondered if it would be possible to build some kind of proxy?? hardware/software to solve this problem. Maybe some combination of tx/rx modules for old and new networks. Maybe use a SDR for the old tx/rx module as it would only need to tx a short distance to the old phone
You could, but if you're doing that, it would be far easier to just build your own device that does what you want and nothing else. You can put together a ZeroPhone or something for about the same complexity as a network proxy device that might have to do things with group texts to make them look valid to an ancient device.
And then you're carrying an ancient device plus another proxy. At some point, it's easier to just buy a halfway recent flip phone and use it until it dies.
Technically possible, sure. Practical, I don't think nearly as much.
Plus, if you're actually rebroadcasting an old signal over the air, then the FCC gets involved. And that's an entire new can of worms.
