Here in Australia, POTS lines have been completely decommissioned, UK will be switched off by end of 2025 and I'm assuming there's similar timelines in lots of other countries.
They're on the way out in France, too. New buildings don't get copper anymore, only fiber.
However, as I understand it, at least for commercial use, the phone company provides some kind of box that has battery-backing so it can provide phone service for a certain duration in case of emergency.
The tricky part with that is that, at least in Canada, the RJ11 ports on the ONT are generally VoIP. They provide the appropriate voltages for a conventional POTS phone to work but digitize & compress the audio and send it along to the Telco as SIP or whatever. That works fine for voice but you're probably going to have a hard time using a conventional POTS modem over that connection. I've never tested it and am honestly pretty curious to see how well/poorly it would work.
However, I’m only familiar with the emergency phone call use case, for which voice is enough. I’m not familiar with any legal obligation to provide data service, so I guess that if you need that, it’s up to you to negotiate SLAs or have multiple providers.
However, as I understand it, at least for commercial use, the phone company provides some kind of box that has battery-backing so it can provide phone service for a certain duration in case of emergency.