The board at /play/ is built for a phone in one hand: large tokens, a dice you tap, no hover menus. Rotate is not required.
Browser
Any current mobile browser. Add the page to your home screen if you like; it still loads from the site. Offline modes need the page loaded once. Online modes need a signal.
Android app
Same rules, extra themes, 3D dice. Not on Play as an installable release yet — download page. When it is, you can use the same optional account as the website.
Small-screen habits
Turn sound off in a quiet carriage. Use a Friends code rather than passing the phone if two of you both have data. If the keyboard covers the Join field, dismiss it before you tap Join.