DUO offers four different methods of authentication:
Push is used when Duo's mobile application sends a push notification to your smartphone. Just tap “Approve” to authenticate.
Passcode (softtoken) from Duo’s mobile application it provides a 6digit unique code to authenticate.
Call is the simple feature where Duo Security calls your device and you authenticate by pressing a key.
SMS is when Duo Security sends a passcode to your device via a text message or SMS. You then authenticate by entering your onetime passcode.