Here's what you do according to a service called Twilio.com:
When you dial (310) 299-8756, press two and choose what artist you’d like to leave a message for. For Beastie Boys, press one. For Trent Reznor, press two. Once you leave a message, press pound and stay on the line. You’ll then hear the new track you just unlocked. After you hang up, you’ll get a text via Twilio SMS with a link to download the track you just heard.
(Via Consequence of Sound)