Thread non-sequitur... remember World War Z, the movie very unlike the book? In one scene the guy is trying to be quiet around zombies and he gets a phone call that wakes them all up.
While kinda silly, this practical concern was something that was actually pretty relevant when started parsing out the consequences of sending Infallible Messenger to spies and such. It's really not a great spell to use unless you have a dedicated 'telegraph operator' who doesn't mind having sprites pop up all the time to tell him things.
This also lead to the creation of another spell which was a kind of Sprite secretary, that would intercept the messenger sprites and make them wait quietly until the master was ready to see them.
If I remember correctly, I believe there was even a hearthstone in one of the books that functioned like that, allowing you to avoid getting the messages until you wanted to accept them.
As another aside, I did once use Infallible Messenger along those lines.
Our characters were being followed by a Night Caste Exalt who had tangled with our characters several times in the past. He was planning to kill us, but fortunately he had told us his name at one point. We finally tracked him down to a large restaurant and we knew he was somewhere inside. But, he was a master of disguise so it was impossible to tell who he was.
So my character cast Infallible Messenger and send him a message - when it would get to him it would start yelling really loud, "I'm right here! I'm the Solar! Look at me! Come and kill me!"
The ST shot me a dirty look, read the spell and sighed. "Wow. That's pretty freaking cheap."
So the little cherub flew over to the Day Caste and totally blew his cover.