Sure, there's electricity in there. But you're only getting a few milliamps (about 30) at up to 70 volts. So basically, there isn't enough power in there to run anything. Hell, my cell phone charger needs almost 90 milliamps to run. And that's at your standard 120v.

So basically, it's a neat parlor trick, but won't do you any good for running something over a couple of LEDs.

I'm not sure that's true, MGR -- the original phones used the 'ringer' wire to power electromagnets that rang the phone bells. Those would have drawn a lot more than 30mA, and as far as I know the system is still the same; you can plug in an old rotary phone to a modern phone jack and it will still ring. Also, a couple of times I've been wiring a jack in when a call came through, and it gave me quite a jolt.

I agree, you can't draw too much, mostly because the wires would overheat (and they must have some kind of circuit protection as well) but I wouldn't be surprised if you could pull an amp or two.

After some digging around, the most I've seen reported (none scientific) is about 80mA. Also, I found a website that sells things that are made to run off the lines.
http://www.sandman.com/telco.html
If it's fake, I can't tell, but the prices are ridiculously high.

I'm no electrical engineer, though. Just had a license as an electrician for two years, but I only did some basic stuff. So you may be right, but I'm stikin' to my theory

Hmm, now I'm very tempted to take a resistor and multimeter to my phone jacks.... I'm just not sure what the consequences are of overload!

[update] OK, I found an FCC spec (47CFR Part 68) that gives testing impedance for ringer circuits; maximum current for testing is 93mA. Assuming that any overload protection would be set at least 20% higher than that, you could probably get 100mA. But you're right, MGR, that's only ~7W, not useful for very much. And I'm not going to go so far as to calculate the heating of the wires at that power.

This is for DC sources.

I think you mean DC sinks.