We’ve actually seen evidence of this back in June, although a recent APK teardown by the folks at Android Police have shown that Google is closer to making the integration a reality. For those unfamiliar, Allo and Duo are communications apps launched by Google last year, with Allo being a messaging app while Duo is a video calling app.
Google has so far tried to integrate both apps within the Android system as much as possible, making it clear that these are both apps that the company wants Android users to start using as their de facto communications tools. In fact Google has made some updates to Allo to allow users to start Duo calls.
That being said, it is unclear if this integration will indeed make the next update for Google Phone, or if the code could be there as part of a future release, but for Duo users out there this could be an update to keep an eye out for.