[iwwr]

Is this a good illustration of the phenomenon?

http://i.imgur.com/cDgjD.jpg

The glass is the antenna and the shape (here a circle) radius at a given point is the signal strength at that point.

The glass is exploiting some anisotropies(?) in the cell phone signals. i.e. the red arc is somehow bigger than the black arc. I am using the circle as a way to depict an isotropic signal. If the signal is not isotropic, the shape would be somewhat more bulged in the left region (pear-shaped).

Sorry, IANAP, if someone can explain it better...

[zinkem]

I think what OP is referring to is when light hits glass, some times it is reflected and sometimes it will move through the glass, depending on the angle it makes with the surface of the glass. So the signal is probably bouncing around inside the glass so the phone is able to pick it up.