Damn, I guess I unwittingly revealed my phonecall preferences. :)

Edit: Isn't there some kind of wake-on-lan on arm chips that the Baseband OS could use?

Yes. Additionally, why would you want this specifically? I think OP just wants better battery life?

If you're going as basic as OP wants you can get 10+ days on a typical phone battery regardless, just by implementing battery saver. I've done it with both smart and feature phones.

Not necessarily, if the cell modem stays powered on and can "wake" the application processor into booting when a call/sms is received.

That's what I had in mind. (See my edit above from earlier.)

Especially since both Pinephone and Librem devs are essentially dumpster diving for their hardware. Even Dell with all its resources couldn't get fast suspend on Ubuntu working (or whatever it's called). And that was just x86_64. But even I know how to power an RPI on/off! (I can even run the systemd thingy that reports startup times for various processes, which already puts us years ahead of the competition.)

If we work smart I honestly believe we can have a Linux device with the feature set and battery life of my LG VX5200 by the year 2030.

Edit: here's a sheet on that wonderful little indestructable device:

https://www.lg.com/us/mobile-phones/pdf/VX5200_Datasheet.pdf

Notice the support for "Mobile Web 2.0!" I don't believe that included SVG-- so already we're even closer to our goal!!!