Users can use Duo video call in the middle of the phone

Google Duo
Google began rolling out new features in its mobile apps, allowing Duo video calls to start when making regular phone calls. Google Phone app users will find new buttons on call options in the Phone app. The new button is labeled as a call and is represented by a Duo icon. Pressing the button will stop regular phone calls, and launch the Duo app on each device that is communicating.

If the recipient is incompatible or does not want to appear in the camera for different reasons, the video call may be rejected. However, there is no special dial tone that indicates the caller is trying to contact via video.
This causes the recipient to pay attention to the screen or will skip the video call. If a video call is denied or not connected, a standard phone call screen display will reappear.
In addition, the Duo app will not switch to video view if the receiving recipient does not unlock the device. If the phone is locked, a small message appears telling you that the device must be unlocked to start a video call.
The caller will receive a message mentioning that the recipient is not available to receive video calls. Calls that switch to video chat on Duo will appear twice in the phone call list. One as an original phone call, and another as a call to video chat Duo. This new feature will start rolling in to users of the Google Phone app, and will be rolled out to all users of the app regularly.