WMG / Adele

In "Hello", either the narrator or the person she's addressing is actually dead.

"The other side" is a common phrase to describe the afterlife, so it might not be just be physical distance between them. The narrator had a lover who she hurt; they broke up and stopped talking to each other, and he passed on without her ever getting to apologize. Either she's been calling his old phone number, which is still in service, to leave voicemail to try and ease her mind whenever she thinks about him, or she's not literally calling him, just thinking about what she would say if she had the chance. Alternately, she herself is a ghost and apologizing to him is her Unfinished Business - she's "calling" him from beyond the grave, trying to get his forgiveness so she can move on. The biggest obstacle to the latter interpretation is the lyric "I'm in California", which clearly places her on the physical plane... But that could be some odd metaphor for the afterlife or just a reference to where she's buried.