According to a Google blog post, the company is rolling out integrated video calling to first-generation Pixel, Android One and Nexus devices. It will also show up on the Pixel 2. "We're working with our carrier and device partners to bring this experience to more Android devices over time," wrote product manager Jan Jedrzejowicz. If you (and the call recipient) have a carrier that supports ViLTE, your video calls will go through that service. If not, Duo will connect you to anyone with the app installed.