Duo is Google's video calling app, and it looks like the tech giant wants to spread the word about it. According to Android Police, Duo users can now call people who don't have the app installed and who haven't registered with the service. It works like any other Duo communication, except that at the end of any call, recipients who don't have the app installed will then be prompted to install Duo. They also have the option to decline future Duo calls from that person. We've contacted Google for confirmation.