there are several different ways to do something that looks like this, it all depends on what features you want exactly and how much work you (as the developer) are willing to put into it.
for exanple one way to handle this would be to use show picture instead of show text - make the messages exactly like you want them in GIMP or Photoshop or whatever image program you like, including a real phone frame with a hand holding it as part of the picture.
But that would require a lot of pictures...
another way would be to use a messaging system that allows you to change size and position of the show text messages, then use a picture of a phone as a background and that show text with border set to invisible.
If you want a plugin to handle everything for you, you'll need to give better specification.
actually, are there any JRPG type games (I guess turn based, so not ARPGs like say, Vagrant Story) where you have only one party member for the majority of the game? (I wonder if I'm forgetting something obvious.)