Back to the main interface, once you tap on Once the call launches, like any video calling service, your front camera view switches to a small window and the rest of your screen is overtaken by your recipient's stream.
Compared to Facetime, Skype, Hangouts, and many other services, setup is faster and simpler, and usage afterward is more straightforward.
And yes phone numbers might be ripe for abuse and spam, but so are usernames and email addresses. I'd wager the reason is that Google wanted to do less, but do it better. Launch the app and you're greeted with your face from the front camera on your phone (we've already established it might not be sexy), a white bottom overlay with a button, and an overflow menu on the top right.
Once that foundation is built and trust is gained with users, I'm sure Duo will start adding more traditional options and filling the gaps for a more demanding user base.
Simplicity is the name of the game with Duo and that starts with the setup.
You can take and make calls with its built-in speaker.
You can also check voicemessages and hear the alarm you set.
Thankfully, Knock Knock doesn't work for contacts you don't have in your address book - a nice way to avoid unsolicited images from people you don't know.
And it can also be disabled, but keep in mind that if you turn it off, you won't send your stream to your contacts but you also won't get theirs when they call, regardless of whether or not they have it enabled or disabled.
Once you've made a couple of calls, the bottom overlay will start populating with your most recent contacts. (You can't see me now, but you will in a while.) If you're a geek like me, the first thing you'll look for are the settings.
They're nested in the overflow menu, along with Help and Feedback, and there aren't many.
If you're on the receiving end of a Duo call, you'll see the recipient before picking up if you both have Knock Knock enabled, and Duo will also show you which network it's currently using for its call.