Duo's call history integration could be coming soon, as it looks like the app is starting to prompt users with an alert when they open up the application that they can have their Duo call history show up in their regular call history within the Phone app. The prompt tells users that they can keep track of their Duo call history if they let Duo manage and make phone calls, and then gives users the option to allow the management to proceed or to decline the option. Presumably if you decline it you should be able to go back into Duo's settings and enable the integration at a later point if you change your mind.
This may or may not be a feature that's actually going live as part of an update. It doesn't seem to be showing up for everyone just yet so it could be something that Google is either rolling out in stages across the range of users who have Duo installed or it could be something that it has to turn on server side, though even in that scenario Google could be turning the feature on in stages as it wouldn't likely be rolling it out to all users at once. It's also worth mentioning that even with this prompt showing up for some, making calls in Duo doesn't seem to be populating the Duo call history within the Phone app's call history just yet.
This is also the first visible sign it looks like of the feature going live for some as it was discovered to potentially be coming back towards the beginning of May. The notion of Duo's request to manage and make calls might also suggest that Duo will be getting the ability to simply make voice calls if you wish, though Google hasn't yet mentioned this as being a future capability, and so far this seems to be the only sign that it could become a usable feature at some point. For now it seems that this may be the slow start of Google implementing this new function and if that's the case then Duo may be able to have its call history show up in the default call history fairly soon.