Google has finally enabled a dedicated ‘Video call’ option in its Phone app. The option in question shows up while you make a normal call, and tapping on it launches the Duo app, switching you to a video call.
Tapping this new option during an ongoing call launches the caller’s video camera. On the receiver’s end, Duo is launched as well (provided their device is unlocked). Of course, triggering this whole process means the ongoing normal call is disconnected.