Any phone that can send and receive text messages. Most modern Android and iPhone phones support RCS natively, which gives you buttons, images, and rich cards. Older phones typically use SMS, but you should be able to upgrade in settings. Go to your phone settings -> search RCS -> turn it on. If your phone doesn't support RCS, Slothwise automatically falls back to SMS with the same commands and features, text instead of buttons, but you still receive images (charts, quiz results) via MMS. You can also use the Slothwise app on iOS or Android (currently in the waitlist mode).