This was happening in the last six months leading up to the movie, so that's probably all there is. It's fun to speculate what other characters could have done messages though...
"Hi! This is Wedge Antilles, doomed to live forever in Luke Skywalker's shadow. What's my secret to surviving battles with the Empire, when all my fellow pilots seem to end up frozen chunks of meat orbiting Yavin? Find out when the Empire Strikes Back!"
"This is Boba Fett. I was the only good thing in the Holiday Special. Han Solo won't escape me, nor the wrath of Jabba the Hutt, when the Empire Strikes Back! And then I'll collect the bounty on Kenner for ruining my action figure. Enjoy my silky voice while you can, because someday, I'll be dubbed over by some guy from New Zealand."