These days POTS lines are usually only used for last mile communications so the calls get converted to VoIP on the telco side. Basically it's for backwards compatibility — the phone lines are already there, a lot of people have phone wiring in their houses and no configuration is required on the consumer's end.

In general the days of having direct electrical connections between two distant telephones are long gone. The telco companies scrapped it when they realized that they could trunk the phone calls from a local branch to the central office using PCM streams over a single cable.

Metallic path between two stations that weren't terminated in the same CO has been dead for a long time! I suspect nowadays you're unlikely to have metallic path outside the frame you land on, if that, unless you're paying for dry pairs.

The POTS lines were largely for goofing around, though I did use a FXO port to bring in the local POTS line. Mostly I used the FXS ports to interface a 1A2 KSU to Asterisk to run my old WE 2500 series key station.

90% of the traffic was handled over SIP or IAX to desk sets or ATAs.

The H3 has an M.2 slot and a separate emmc slot. So if you really wanted to use that card perhaps you could use an M.2 to PCIe connector and then connect that in turn to a PCIe to PCI adapter/riser. Power might be an issue though.