In theory, yes -- as long as the unit does not take much power.

The down side is that if you use too many of them, your phones may stop working. Also, if too many people use them, the phone company may want to have a talk with you.

Another thing to consider is that when the phone rings, a lot of voltage is generated (over 60V, I believe). So, if you just go around plugging random things into your phone jacks, they may be fried the next time that somebody calls. Whatever you plug in has to be designed to handle these transients.

So, while this unit may work, I rather doubt its usefullness under ordinary circumstances. If it can be powered off of the phone line, you are saving yourself maybe $1 of electricity over using a similar light running off of 120V.

I would also not buy more than one of them. For emergency use, one of them may be quite useful. But as you plug more in, the more you load down the phone power supply.