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Wouldn't put more than like 5V DC on it, but you could use it to put power to some low power USB toys.
It's not the voltage you need to focus on. It's the current.
Analog telephone wiring used 90v @20hz to ring the phones. Off-hook, they were at 9vdc, which rose to 48vdc on-hook. The insulation can handle potentials much higher than 5v.
But, telephone wires are typically 26awg to 22awg, which are only rated to .361 to .92 amps for power transmission. Pull too much current, and you risk melting the insulation and starting a fire.
Since DC power (watts) are volts times amps, to keep the amperage low, you will want to keep the voltage as high as the insulation will tolerate.
Telephone wiring is very similar to (and may even be carried on) cat5/6 cabling. There are formal standards for using cat5/6 cabling for power transmission: Power-Over-Ethernet standards 802.3af, 802.3at and 802.3bt. These standards call for 44 to 60 volt power injectors, and up to 15.4 watts per wire pair. If I were going to jury rig household telephone wiring for power transmission, I would use those standards as a guide.
You should use a current-limited power supply, to keep your current below 0.361A, or you risk melting the insulation and starting a fire.
Very well thought out, thank you for this.
If it's still hooked up, it might be useful as an emergency power source for keeping cell phones and USB power banks charged in the event of a long-term power outage.
https://www.instructables.com/How-to-Get-Emergency-Power-from-a-Phone-Line/
My family was without power for three days recently, and about a week a few years ago.
I have solar so during the day I don’t need emergency power. That’s good to know though
I can confirm those numbers, I once was touching the terminal when a call came through. Shocking experience, would not recommend.
In addition to this, the voltage drop from one end of the wire to the other would probably make a 5V input useless to begin with when you only get 1.5V out on the other end. Higher input voltage doesn't suffer quite as badly while also helping to keep the current low like you mentioned.