Ma Bell used -48v DC in the US, which was backed by a giant lead acid battery at the local exchange. It was quite common to have ADSL circuits working during major power outages, so long as the modem and servers were on redundant power. It might still be worth coming up with a USB charger circuit, but I assume current draw is metered.
I’ve seen COTS adapters which connect to a standard phone jack with pass through and provide one USB port, advertised as an emergency/backup power. I don’t know how much power they can draw, however.
The one I’m familiar with is called:
VR3 (brand) Phone Jack Powered Emergency Night Light & Charger (model: PJNL-USB)
