How about a video baby monitor. Set up the phone to show onscreen the notification and keep the screen on when on the charger then use a baby video monitor so you can see the screen in another room.
I'd like to place my iphone in a spot near a below-street level window where it can pick up a signal, and sit at a distance from it in the underground room where there's no service. I need a device that the phone can communicate with (perhaps bluetooth?) so I know if I got a text message. Any suggestions?