Odds of dial-up working when cellular doesn’t are low. After hurricane Maria in Puerto Rico, cell service came back in most places before landlines. That dial-tone no longer means connectivity to a telephone switch. Analog calls are digitized and carried on the same high-speed long-haul digital networks as everything else. The only thing in POTS that’s still analog is the “last mile” wiring from the phone jack to the nearest mux.

The best disaster preparedness is to assume a complete loss of internet access. Read some of the stories of people trying to keep data centers running in New Orleans after Katrina and you’ll see why in a large scale disaster, the data centers will be down within days and connectivity won’t matter. Scrap the old modems and get a HAM radio for comms and an “Internet in a Box” (https://internet-in-a-box.org/) - or similar setup - for off-line information/entertainment access.