Update to Duo adds video calls to your phone's native call log
Update to version 14 of Duo from the Google Play Store
The latest update to Duo, which is version 14, allows users to add video call history to their handset's native phone call log. All users have to do is allow their phone access to the Duo call history one time, and after that all Duo video calls will show up together with regular calls. A small camcorder icon under the phone number will denote a video call.
The update to version 14 of Duo will come you via the Google Play Store. If it has yet to arrive, open the Play Store app and click on the hamburger menu inside the Google Play search bar on the left side. Tap on My apps & games. Scroll to Google Duo and click on update.
source: Duo (Android|iOS) via Google 9to5