The problem is you either need two lines with your carrier or swap physical sims. Swapping physical sims is inconvenient and means you cant use an esim.
Probably what I'd rather have is a phone that has a low power low function mode, that I can configure what apps are available, and then a regular mode where everything is available. But to get to it, I need to convince a LLM equivalent of Seinfeld's soup nazi why I need it.
Isn't this literally already built into Android? Extreme Battery Saver effectively turns your phone into a feature phone, disabling every app except Clock, Phone, Messages, Settings, the Play Store, and your camera. And you can add other essential apps to the whitelist (e.g. I have my note taking, ride sharing, banking/wallet, calendar and TOTP apps enabled, and my music player if I'm on the go). It even degrades the phone's performance to match lol, and the added battery life you get from it is fantastic.
Might be a Pixel-only thing now that I think of it...then again I'm pretty sure other OEMs like Samsung would have their own versions
I have a agm m8 flip that I have at all times for calls and text. And I have a pixel running grapheneos that I use more like a laptop / when needed.
Living without a smartphone isn't possible. But not using one for two or three days is.
Most banking apps require a smartphone.
If I'm out an about I will generally not have a smart phone unless I'm making an unfamiliar journey or something like that.
I've got through quite a few dumbphones, including the nokie 8110 (banana phone, but rooted with app store removed), Nokia 2660, cat s22 flip, agm m6 But all were sub par in some way. The m8 is the first that has ticked all the boxes for me. I love it.
Voice comms go over a VOIP service such as Jitsi or whatevs.
Carry the VOIP device you want to use, whether a minimal comms-only device, or a more capable tablet or laptop which you can talk to.
A device which talks to a headset or transport-based option (e.g., your car's Bluetooth system) might also be convenient. The idea that you need a full-compute device with a high-res screen just to make voice calls is ... somewhat silly.
The main concern I have with this is the level of phone spam either now present or in the likely future. You'll want a comms system which filters out unknown and/or suspect calls. PSTN, of any stripe (POTS, mobile, VOIP) isn't cutting it any more.