Anybody knows what the song, "The Call" by Regina Spektor, is about?

I think you're right. I know that the song was written for the Prince Caspian movie, and it was played when the children had to leave Narnia, knowing that they can't come back again like they used to...I never got to read the last book, so I'm not sure if the children ever were able to be placed back with Narnia, but it's the general idea of being able to say goodbye, but also realizing that it'll never finalize a departure.