I came here to write almost the same thing
#1s was the first Mariah album I bought, and I loved Mariah and Bryan singing together on Whenever You Call. He has a truly beautiful voice and it blends really well with her. So, in a way, it brings me back memories and feelings from 1998/1999. Ithink this is one of her best ballads. Great production, lyrics, and use of her range.
No, I don't think it was meant to be a duet, otherwise it would have been recorded as one, since Mariah, at that point, was "allowed" to record with other people. "Butterfly" is maybe the best example of Mariah writing and singing something that someone else was supposed to say. "Spread your wings and prepare to fly, for you have become a butterly. Fly abandonedly into the sun. I fyou should return to me, we truly were meant to be. So spread your wings and fly, butterfly" was what Mariah wanted Mottola telling her, and she is saying it to herself, with no male voice. So, "Whenever You Call" would not be the only song that could work with two people singing, but she is alone.