There exists a species known as "POTS Replacement" router.
It refers to, in effect, a cellphone that you screw to the wall; it has an RJ11 jack for a POTS phone. It's used where you can no longer order a POTS line from the LEC, such as in much of Verizon FIOS territory.
VZ tried to foist off an older version called Voice Link on some coastal NJ/NY areas where their cable plant was destroyed by Sandy; it backfired badly for residence use.
But current products do well serving existing dialup alarm systems, and NFPA fire alarm systems. Another term is "Elevator Phone" as elevators require emergency phones.
I wonder what, if any, models CDT readers have experience with? I seek one with:
a) 1 Line, RJ-11 jack b) The carrier will be AT&T Wireless c) LTE Voice {and maybe even 5GeeWhiz...} d) It needs jacks for external outdoor antennas e) And 12/24VDC power
It's not for a fire alarm, which requires NFPA approval, just voice.