According to Android Police, the Duo button is showing up in some users' regular phone dialer app. When you tap the video call button, your phone launches the Duo app and activates your camera. The person you're calling can then accept or decline the incoming video call. Android Police notes that the initial phone call will disconnect when the Duo video chat takes over, and that you'll have two entries in your call log — one for the initial phone call and one for the video call.