Actually I think it might work ... sort of
Assuming the phone antenna is pretty much omnidirectional most of the signal is going nowhere useful, so you can "harvest" around half of that with an inductor on one side, and subsequently you'll have to point the other side towards the mast. The only problem is that you'd want to put the inductor on the side that's against the user's head (because the signal in that direction is attenuated anyway) which won't work too well with the screen. But if you're willing to put up with the hassle of a directional phone you might as well engineer one with a directional antenna, that wastes a lot less power transmitting in the first place.