During Google's reveal of the Pixel 2 on October 4th, we spotted a new video call button appearing on the Phone's in-call screen. Tapping on it was meant to switch both members to a video call using Google Duo. Google later added details to support articles that implied this functionality was already live, but the button remained hidden through a few updates to Duo and a couple to the Phone app. That changes today as the video call button is now going live in Google's Phone app when you're calling a number registered with Duo.
Left: in-call screen. Center: starting video call. Right: recipient switched to Duo.
The switch to a Duo call is mostly seamless. After hitting the button, both ends of the call will launch the Duo app with the caller's camera being automatically active with the Knock-Knock feature. The recipient can choose to accept the video call or decline. There aren't any audio tones or clues that this is happening, so it's easy for the recipient to miss if they're not watching the screen. Just remember to say something if you're going to use this.
Once the video call is connected, it will automatically hang up the original phone call and continue on with Duo. In my quick tests, I didn't even notice any gap in the audio since the video call takes over instantly.
If the recipient doesn't accept a video call, it will eventually time out and return to the phone's in-call screen. Also, Duo requires that the phone be unlocked before it can switch to a video call. That means that a recipient may have to actually leave the in-call screen to reach the lockscreen and enter a PIN, pattern, or even just swipe to bypass it. Until that's done, the recipient only gets a subtle toast message that video calls can't start while the phone is locked, and the caller gets an unhelpful message that the other person is 'unavailable.' This should be fixed.
Switching to a video call in this way will leave two entries in your call log, one from the phone call and the other is from Duo.
The video call button doesn't appear to be tied to any specific versions of either Phone or Duo, so it was probably activated remotely and may not be available to all users yet. It worked for me when I called from my Pixel 2 with v14 of the Google Phone app and v24 of Duo. Naturally, the video call button isn't available to anybody that doesn't use Google's Phone app.