I have one of those, too. But it won’t do me any good in a long-term power disruption scenario, because my landline is no longer a straight shot to the CO. Instead, it terminates in a neighborhood junction box, which contains active equipment that will stop working when the battery backup craps out.
The old phone network had many shortcomings – but reliability wasn’t one of them.
We’ve traded away resiliency for “efficiency” in so many areas, usually without realizing it, until something goes wrong.