Ah, a nice riddle.
There's a lot of different ways of communicating.
Your phone uses a lot of them.
Most of them are one-way.
You have to put the unit on your screen i order to synchronize.
The phone has a speaker to communicate you're being called or sent a message (the speaker sounds a ringtone).
That is a one way communication mode.
The phone can show you what's going on by displaying a message on its screen, also one way communication.
You can touch the screen, interacting with its contents.
That is "1.7 way" communication, the touch part is way different from the display part yet combined they get close to 2 way communication.
As the device has to be placed on the screen, it is likely that is what it communicates with.
The device could recieve data by screenpixels and an optical sensor.
It could then interfere (disrupt) the capacitance and so emulate touching the capacitive touch screen.
This would need the user to initiate the sync on the phone (start the app), and the phone needs to be equipped with a capacitive touch screen (resistive touch needs actual force, capacitive doesn't).
This doesn't get you a high bandwith, but the amount of data to be moved for this type of device is quite moderate so this could just work fine.
So i guess this might be the way this works.