I have an old-style hard-wired phone to supplement my cordless phones. It still died after a few hours.
Before fiber, the copper phone lines carried power as well as signal. With the switch over to fiber, this is no longer the case.
I think what has happened, is that those copper wires used to go all the way back to a telco central office where there were big battery strings and generators to run an old-school switch. Now, those copper wires go back to a big tan box somewhere fairly nearby, and turn into fiber optics and VOIP. There are a few hours of battery there, and then you’ve got nothing.