Technically, if it's a land line port and still connected to an exchange that hasn't gone completely VoIP (that's a thing where I am), it might actually be possible to build a charger module that plugs into that port.
Would it be worth it, though? ... No.
Low power is supplied over old land-lines for the purposes of making telephones ring and powering other handset bits and pieces, within reason of course. Using it for anything else is undoubtedly illegal as phone lines aren't rated for huge power draws.
(If you're interested, there are videos online where people have hooked up LED lamps etc.)
But, let's say that module existed and was legal. Your niece still wouldn't be happy with it.
To avoid burning out to the telephone line, any such device would have to be a r e a l l y s l o w trickle charge.
I wouldn't even think about it for emergency power outages. A battery backup is a better option.
RS232 is functionally immortal. Its market share in the niches it fills has never -- and I'd argue will never -- go away, or even shrink all that much. It's like those lobsters that don't age at all but if we splice the genes that do that into humans it gives us cancer.
Could work in theory. Back then there it had sonething like 40 volts going through the line and you needed some decent power to make the bell in the phone ring.
But I don't know if that's still in use these days.
Completely off topic to the thread, but you just reminded me of a time I snuck onto a movie set and got to actually do that. I posed as a driver for the car company and got to start/drive one of those bad boys with the hand crank. Inside was all switches too which was wild. The most uncomfortable ride of my life.
I remember time before time existed, on cold winter mornings we'd gather around in front of our Model T and turned that crank just below the grill.
Back then we also had to crank-watch television in black and white, uphill, backwards both ways, during snowstorms... just to get to bed.
The kid was blown away by the modem. For those who don't know it's a cradle type dial up modem where you place the (land line) phone on a receiver instead of plugging the computer into the cat4. You could get up to 150 bits per second on one of those bad boys.
That's about the speed you can read text...it's why pre-internet sites like BBSes weren't all flashy, you had to keep it loadable. Actual downloads you would plan overnight and hope you didn't lose connection. The first big breakthrough was resumable downloading where you left off. Huge.
Don’t know what the equivalent is today. But that one solidarity Commodore Pet, green screen glowing in the corner of the classroom, will always have a special 32kb space in my heart.
You young whipper snappers and your fancy DOS terminals.....10,000 years ago I sat at a teletype terminal and tried to learn to program in BASIC. Oregon Trail and Missile Attack are a whole 'nother experience when done by printed media only.
I can still hear the sound of the teletype clacking.....