Re: "sealed lead-acid battery-backed up 12v power supplies out there"
But this is where regulation plays an important part, ie any safety-of-life stuff. A bit like the Capita/Gas story where we decided that having stuff on the end of a piped in flammable/explosive gas being certified and installed by competant tradespeople is a good thing. Even if competence may not extend to the entity managing parts of that process.
Hence regulatory approaches to providing 'phone' services. And that's been more onerous around the critical functions, like being able to make emergency calls. And also extends into other 'lifeline' medical, alerting or alarm services. Some of those are less regulated, but still a concern even if it's to avoid reputational damage when one of your customers has an avoidable death. Or like the Santa Clara fire story, customers don't understand the limitations of the service.
If phone services stay regulated, then capabilities can be defined, and sanctions applied to breaches. So a phone line must be capable of making an emergency call, and responders must have a clear indication where the caller is, and be able to hear them. So then UPS requirements can be defined to cover power loss, codecs for speech quality, and any signalling to give a location. Plus there's some new-tech functionality that's possible, ie using something like Ethernet's 'Dying Gasp' to signal faults on critical lines. And there can also be issues with integrating Emergency Broadcast Networks to warn of impending events like tsunamis, earthquakes etc.
That may mean costs, ie installing LTE with UPS capability, then maintaining it. With POTS, batteries and charging are centralised and phones can be line powered. But this is also where regulatory gamesmanship can come in. So in the UK and other countries, access to copper phone lines is possible via LLU. Jumper the copper across to a competitor and they take responsibility. It's also possible to unbundle xDSL given it's usually a simple notch filter that allows the 'voice' part to be extracted.
But.. that also limits capacity on an xDSL line because some of the copper's bandwidth is reserved, plus there's potential to interfere with non-DSL services running along copper cables. Going to an all-packet network means making sure all that stuff still works. Some countries regulate this, eg in Ireland, Eircomm's wholesale DSL service includes a priority channel intended for VoIP/voice with defined interconnects.
In practice, that can be a little clunky, and customers not understand the implications. So business wants MLPS/IP trunks so they can save money on their phone lines. But then we have to explain that they really should keep 1 POTS line per site with an ATA connected to their call mangler and emergency calls routed to that ATA. It was depressing the number of times I spoke with customers who wanted to hang everything in remote offices off an xDSL line.. Including their mobile gateways, because mobile reception was poor. Best way to resolve that one was explaining potential director's liability, and their insurance wouldn't generally cover negligence.
Downside is good'ol capitalism. Ignore regulatory stuff, and you can make more money. Send the customer a sticker saying 'This won't work in a power cut, good luck!' is cheaper than supplying a UPS. And if voice is unregulated, you can also save on the stickers. Or, it can mean not paying USO charges that are meant to subsidise rural services. Or, if regulations permit LLU for copper, switching to fibre means dodging that regulation, and preventing competitors from accessing your customers.