You can now use Google Duo on the web without needing to link your phone number. As Android Police reports, it isn't clear when Google made the change. But now you can make and take calls on the web. It doesn't seem to be fully working for G Suite accounts. You can place calls, but G Suite users can't receive them for now. If you are using a standard Google account, just head over to duo.google.com and sign in (you don't need to have a linked phone number). You will be directed to the main Duo site where you can search through your existing contacts to place either a video or audio call. As for those with a G Suite account, you'll hit a roadblock here as the site will ask you for a phone number for verification before you can proceed.
The biggest issue you might encounter is that with email-only Duo accounts, you will not see those within your contact list who are email-only, too. You can only call them if they appear in your recent log or notifications list. Duo will also annoy you through a notification to add your phone number. But other than that, you will still be able to make and take calls. The change appears to be web-only, and the company hasn't updated the support documentation to talk about this feature yet.