When Google released the Home Hub last year it (sort of) introduced the ability for Google Home type devices to make Duo calls without the video component, since the Home Hub lacks a camera. The screen can make a call and deliver audio to someone else, they just won't be able to see you on the call.
That feature is finally making its way over to regular Google Home speakers in a slightly more limited capacity.
Google hasn't made an official announcement about this yet, but users have noticed the setup process for a Google Home Mini prompting them about the ability to make and receive Duo calls on their speaker. As of right now it seems like it's only really working on Google Home Mini devices, with many users reporting the calls fail on a bigger Google Home, and it is limited to calls within your house.
It would make more sense for this to eventually be able to make calls to other users, since otherwise it's just a beefed up intercom system, but that may eventually come after it leaves this in-between available-but-not-released state.
source: Android Police
