There was a reason for Rean to not try and hide the existence of their Communication System from the Zasfins. Simply put, it would be impossible to hide it in the first place. This was something that would occupy an entire continent. How would they keep it a secret with tens of millions of people using the devices? That would be ridiculous. Not to mention that the Zasfins would be idiots if they didn't convince some ex-slaves to deliver information to them through whatever methods.
Since that was the case, Rean laid everything out in the open from the very start. 'Don't worry, this communication system created by my sect will be available for your cities as well. After all, it would make things simple when negotiating with both parties. Now then, please wait in the tower until the representatives of the cities come and talk with you. I'll be there as well when that time comes.'