and SMS itself hasn’t always been available during major disasters.

Neither has running water or electricity. And SMS isn't actually the last fall-back (over here), that's FM radio which has better reach and crank-powered receivers start at like 10 bucks. Also there's a ton of generator-powered receivers around (called cars). Oh, dang, no, that's not actually the last fall-back that'd be megaphone trucks and cars practically all emergency service vehicles have some kind of PA system.

Solar storm killed the electrics of the new vehicles? There's a 60-year old Unimog still standing around getting moved once a year to keep it operational and I bet you'll find an analog megaphone in storage somewhere. It's astonishing how little stuff gets thrown away, we once stumbled across a stash of field telephones, half of them with swastikas ground off, the others still intact. Those require a crank and a copper cable to operate, nothing more. We used them to organise parking for a summer camp before the days of mobile flatrates.

The actual upside of plain ole GSM is that practically everyone carries around a receiver all the time, and there's reception literally everywhere. Better reach and better signal bandwidth than sirens, though of course nothing beats the oh fuck oh fuck hear it in your marrow aspect of sirens.

Catastrophe relief isn't an area where you ever want to have a single strategy because absolutely nothing is 100% fail-safe. In principle something like TETRA would be better than GSM but civilian phones don't speak it. (TETRA uses mesh networking, you can do direct handset to handset calls, drive around base stations in trucks to extend reach, etc)