At first we tried putting the antenna under the back plate of the phone just the way it was, all floppy-like. It didn't scan. For the reader to induce power flow through the antenna, there can't be a bit metal object right next to it, there needs to be a bit of air space.
We achieved this by taping the chip/antenna to a piece of paper, then building up several layers of packing tape until we could reliably scan the tag from inside the phone. The positioning within the phone can affect scanability too.
Replace the back plate of your phone and enjoy!