After enrolling a mobile device to use for authentication, you have two options to choose from to get authentication notification, Duo Mobile Push or Duo Mobile Passcode. Which one you use depends on your preference and situation. Choose the one that sounds right for you:
Duo Mobile Push allows you to authenticate with just the push of a button and is the easiest form of authentication. It requires either cellular service of a WiFi connection.
Duo Mobile Passcodes are one-time codes generated on your mobile device and can be accessed even without an internet connection or cellular service.