His reaction went something like this: "Who? What? How?" Shelton recalls to PEOPLE of getting the phone call to sing on "I'd Want It to Be You."
"It's still shocking to me," he says. "I don't know how it came down. I don't know how she knows about me, but I'm just glad. I'm just going with it!"
Turns out, he also helped make music history: The album's debut on the top of the charts made Streisand the first act to achieve No. 1 albums in each of the last six decades.
Yet, for Shelton there was something about the experience that was a little unsatisfying. "Ironically enough, she had already done her vocal part and I was really disappointed because when I went to the studio, she wasn't there. I was like, 'Oh, man, we ... that's ... I still get to be on her record with her and that's bad-ass.' "
"And they said, 'No, no, no, no, no. She's here. She's on the phone.' And so there she was on the line, I think she was in London, and listening in to this recording and talking to me!"
So was it nerve-racking? "Hell yes, it was nerve-wracking!" says Shelton. "I remember I kept saying to her, 'Man, thank you. I mean, just thank you for thinking of me.' She couldn't have been more humble and more appreciative for all of us that were part of the record. She was very sweet about it. I don't know, I'm still speechless about it, I really am."
As for his performance in the studio, Shelton thinks he earned the Streisand seal of approval. "She sent me a gift," he says. "She must've really liked it."
And just what does Babs send as a thank-you gift? "She sent me a piece of crystal that was like, 'Thank you, Love Barbra.' I pushed all my awards away and put that in the middle. It was so cool, like, 'See, she knows me. I knew it.' "