The iOS app requires iOS 8.0 or above. As for specific phone models, we recommend the following:
iPhone 5/5s: We're continuing to refine and improve the experience for iOS 9 users, but the Automatic app will run on both iOS 8 and iOS 9 for the iPhone 5/5s. iPhone 6/6+/6s/6s+: We recommend that you use the latest version of iOS 9.
The Android app requires Android 4.0 or above, but we recommend using the most current version.
HTC One: The HTC One requires Android 4.3 or above.