Duo's video call button from the phone app is finally live and is available for use for anyone who already uses Duo or will give Duo a try in the future. To get access to the new feature users will need to have the latest versions of the Google Duo and the Google Phone app installed on their devices, and with that covered both users will also need to have their numbers registered with Duo for the feature to work. This is because when one person taps the Duo video call button during an ongoing phone call it will switch both users to a video call, but it can't do so if only one user has their number registered with the app.
If you have both apps updated to the latest versions this still doesn't seem to guarantee that the Duo video call button will show up in the phone app during a voice call that you already have in place. It's reportedly only showing up for some users at the moment which means that Google probably has to also flip the switch server side for the feature to surface, and Google appears to be doing this in batches since it's not showing up for all users right now.
That makes things a little bit unclear as to how long the wait will be before all users will have access to it, but the silver lining for those who are eager to have their hands on this new feature is that the wait is over at least for it to start going live, and now that it's rolling out to some it shouldn't be too long before it rolls out to a lot more people. If you're the one initiating the Duo call from an existing voice call then the other person will have the opportunity to decline the call if they don't want to partake in a video call, but if they do the voice call will still stay connected until the transition to the Duo video call has been completed. Overall it seems like it will be a pretty seamless process for anyone looking to give this a try.