Here's a bit from the uploader to explain this one, as I was never aware of this at the time (I was too young to comprehend ESB... or my surroundings for that matter):
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). Go to Starwars.com and listen to them yourself without the additonal music.
