This is a sweet acoustic love song sang by a man, presumably to a woman. It isn’t particularly sad from what I remember.
There is something in the lyrics about how nice it is just to hear you say hello, what a difference phone calls make, and “going to my office job and writing love songs for you” (that’s just a paraphrase though, I can’t exactly remember the lyric). I believe the song is pretty recent, and probably an indie artist. I might have been projecting my own emotions and situation onto this song, but from what I remember there was really a focus on just how much it means to hear a simple “hello” form the person you love.
Also, I believe I heard it on a friend’s Spotify account, if that makes any difference.