From the side menu, select “Settings”, scroll down and tap the slider “Phonebook sync” under "Phonebook". You’ll then be prompted to sync your current device to the onoff App.
Warning: This will replace your existing onoff contacts with those on your current device!
On the Android version of the app you can import all your contacts which have a phone number attached to them from your phonebook. After import you can add new numbers to your onoff contact list and periodically it will sync new contacts from your device's phonebook, but your contact list inside the app will be maintained separate to your device's contact list.