It’s rude to not return a call.
John is in a movie theater when Lucy leaves him a voicemail and tells him that the call is really urgent.
John wants to finish watching the movie.
John returns Lucy's call, meets her to help her, and returns to see the movie another day.
John ignores the voicemail on his phone after listening to it, sits back down, and enjoys his film.
Lucy is really appreciative of John's assistance with her problem and pays for his movie tickets.
Lucy feels let down, angry, and has significant difficulty without John returning her call.