To setup Duo, users will need to input their phone number, confirm with a text message, then start using the service. This reflects Google’s aim with Duo which is to create the simplest possible video calling experience.
Duo also comes with a unique feature called “Knock Knock” which shows a live preview of an incoming caller as the call is ringing. This also allows the call to be answered much faster than usual. If this isn’t something you’d like to use, Google does offer the ability to disable it, although it will only work with users who are saved in your contacts.
For the time being Duo will only support 1-on-1 calls which are end-to-end encrypted between users. As previously mentioned the app is now available, however Google’s previoulsy announced “Allo” messaging app doesn’t have a release date at this time.Source: Google