Carlos uploaded the video "BTR Windows Down Party Part 2" where explains how to get this voicemail from Kendall, but yeah. You call 3102564498 like BTR said and you can get KENDALL SCHMIDT on a voicemail. Pretty legit.
He says "Hey, it's Kendall. Sorry if I missed your call. BUT if you're calling about the Windows Down Party, head over to WindowsDownParty.com to RSVP. See you there! WOO HOOO!"