You can only access the mobile version of the VTT if you or the creator of the game you're in has an active subscription. Even if you can access it, I would not recommend using it on a phone; if you've got a tablet, that's much better. The native app is not designed for phones, and so I believe the app store won't let you install it directly. You can probably sideload it onto a phone (I know I sideloaded it onto a Motorola RAZR MAXX HD during the native app's beta testing phase), but YYMV.