We use "cause a scene" to mean that someone creates a situation which gets a lot of attention, or may even be disruptive and embarrassing. "He began to cause a scene there upon his arrival, first through his erratic behavior and random outbursts..." "I needed to sit there calmly and try to be mature even though I wanted to cause a scene..." If you say "Earth to Mag", you mean that Mag is "in outer space" (ie. daydreaming and not paying attention), as if Mag were a space station and you needed to bring her back in contact with Earth (ie. reality) via radio transmission.