Lights like this get posted from time to time. I would not recommend them, not because they are useless, but because the phone system is not meant to power appliances. If too many people use products like this, then the real purpose of the telephone system--to make calls--could be impaired.

The signaling on the POTS (plain old telephone system) relies on changes in line voltage, which is why plugging devices like this into your phone jack is not a good idea.