Also not the most unusual thing I've seen over a RJ11, which includes keyboards, RS-232, other random digital IO and full USB with data. But these are normally on items that are not phones.
It would... if you're willing to just let the thing draw up to 0.1W all the time, and fail catastrophically if anything goes wrong with the zener or the supply is more capable than you anticipated (i.e. you plugged into a PoE jack by accident). This is a very bad idea.
Others using this same enormous, American ISP probably have similarly-equipped gear.
But I must admit that it has also been a very long time since I connected anything there. I don't know if they supply 48v talk battery or not when the modem has a data-only configuration.
Also great for my friends' kids. They can call each other or their grandparents or whoever, without having to touch a cellphone that tries to distract them with YouTube or whatever.
My house uses mostly analog with the Adit 600 (6 lines, 3 using the 70s wiring in the wall, 3 dedicated to specific phones). Everyone else has cheap Polycom desk phones and uses SIP/RTP.
We still have a landline. It's part of the cable package, but it mostly just gets robocalls.
I am not normal.