Hey there,
What are you considering powering off the phoneline? If you are just interested in getting some free electricity, then yes, it is possible, but the amount of electricity that you can use is so small it's almost not worth it.
Think about it, one of the great problems in RE that is discused on this board it that of wire size. We have enough trouble getting the wires to carry the amperage required over the couple hundred feet between mills/panels and the battery bank. Can you imagine how much electricity can be carried over the many miles between you and the exchange over that thin cable. Probably only just enought to ligh a couple LEDs.
And then there are the other problems others mentioned, that I aggree with.
If you just want to do this for fun, and just for the sake of doing it, then mabye you could try lighting a couple LEDs, but nothing more.
Cheers,
Keep having fun!
Stephen.