A companion app to be used with Metal Gear Solid V: The Phantom Pain is now available for free through iTunes and the Google Play Store.
From the official website:
Installing the dedicated Companion App released alongside “METAL GEAR SOLID V: THE PHANTOM PAIN” allows devices such as your smartphone or tablet to act as your very own iDROID.
Ordinarily, bringing up the iDROID in-game blocks most of the on-screen action, so using your device as a second screen means that you can execute your mission while using the various iDROID features on the go.
Aside from using the app as your iDroid, you can also use certain features while you’re away from the game, such as listening to music tracks and cassette tapes obtained in the game.
The app will work with the Xbox One, PlayStation 3 and PlayStation 4 versions of the game.
Source: MGSV Official Website, Web Manual