From Pete Vilmur's intro at SW.com:

"Fans old enough to remember the days before the Empire struck back in 1980 might recall a little-known promotion launched in the months preceding the first Star Wars sequel. As reported in the Spring 1980 issue of Bantha Tracks -- the original Star Wars Fan Club newsletter -- a telephone hotline was set up to allow callers to dial in and hear teasers for The Empire Strikes Back several months before the film's release. In the years since the Bantha Tracks story, fans savvy to the existence of the "Empire Hotline" have sought out recordings of the four messages described in the article, which were actually performed exclusively for the hotline by actors Mark Hamill (Luke Skywalker), Harrison Ford (Han Solo), Anthony Daniels (C-3PO), and James Earl Jones (voice of Darth Vader).

Enter Craig Miller, Lucasfilm's first director of fan relations, who recently uncovered his cassette copy of the phone messages (likely the only copy in existence) and generously shared them with us. To our surprise, the recording revealed five -- not four -- voice performances, including a previously unknown Princess Leia message recorded by Carrie Fisher!"

Click here to listen to the recordings as well as read a bit from Craig Miller as he discusses the inception of the Empire Hotline.