Definitely public for private makes no sense in a million years.
But what to say, what to say?

Option 1
Leave about two minutes of heavy breathing just to mess with future-kind:

"Facinating, Xynoplex456... this message has a new language. We must decipher!"
"Yes, Grtoflex678, and we will not rest until we figure out what this means!"

Option 2
"Call me back when you can"

Option 3
"Is this Dave? Is Dave home?"

Option 4
"Ooops, sorry, wrong number."