The Android version of Google Duo has been quietly updated to support Chromebooks. This is likely in anticipation for the Pixel Slate and its Duo Cam, but it can be installed on clamshell Chromebooks as well.
You can now install Google Duo for voice and video chats on the Acer Chromebook Tab 10 and HP Chromebook X2, even though neither device has a dedicated phone number. Here’s hoping other Chromebooks follow.