I'm actually surprised Rockstar didn't do this themselves.
It's a mod that allows players to control the in-game GTA V mobile phone using your actual real life mobile phone. Genius.
I'm not really sure I'd use it all that much, it's the kind of thing that looks and sounds cool, but might not be too functional in the grand scheme of things. Can't imagine myself wanting to take my hands off the controller in the heat of the moment.
But yeah, this is cool.