The rats in the walls are smarter than they look. They may not talk much, but they listen. And they hear a lot. After all, who suspects that a rat might be listening? And even if they suspect the rat, who imagines that the rat understands?
Of course, to carry information out, the rats need to learn to write, which you already know.
They are almost due for another round of alternatives.
With the right tech selected, the rats can carry their messages further and faster.