Justin Bieber had better get to work putting out new music!

While we await the release of the 21-year-old's newest single, "What Do You Mean" on Friday, nationally syndicated radio host, Elvis Duran, revealed to ET that Justin has A LOT of unreleased music on his phone!

"You know Justin handed me his phone and showed me over 400 songs that he has recorded, that no one has heard," said Elvis. "I said you should release the iPhone, you shouldn't release the album! But he would not let us listen to anything. He wants us to be surprised together. But the new single: fantastic."

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It's been over three years since Justin released his last album, Believe, and Elvis said that the "Baby" singer is a little nervous about putting his music back out there.

"He's honestly nervous, anxious about the new album and how it's going to be received. I mean he really hasn't put himself out there artistically for a while, and he knows that he's changed and his sound has evolved, so now he wants to get it out there and see what we think about it," he said.

Justin actually opened up about his trust issues with Elvis, admitting it's something he has been struggling with.

"Trusting people is hard, especially for me. I've trusted people who I probably shouldn't have trusted before and they've hurt my heart," Justin said bluntly. "I'm a pretty sensitive guy, so I'll lay it all on the line, and they'll kind of pull it. So now I'm figuring out there's certain people I can't do that with."

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And while Elvis has heard the "fantastic" single, "What Do You Mean," what he's really excited for is Justin's follow-up album.

"The only problem I had with the song is it's coming out at the end of summer, and it would have been a great summer song," he admitted. "I am dying to hear the album, that is what I am dying to listen to."

Justin told Ryan Seacrest in July that the new album will be "really uplifting, clear, poppy, but cool."

"The sounds are super expensive. Skrillex is a genius. He's super futuristic and I just love his sound," Bieber explained. "So to be able to incorporate that sound with what I'm doing has been super cool because it's new and fresh. I feel like no one has done it before."

Justin's full interview with Elvis Duran airs on his radio show tomorrow.

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Watch below.