Google Duo lets users make video calls in high definition. It is optimized for low-bandwidth networks. End-to-end encryption is enabled by default. Duo is based on phone numbers, allowing users to call someone from their contact list. The app automatically switches between Wi-Fi and cellular networks. A "Knock Knock" feature lets users see a live preview of the caller before answering. An update in April 2017 lets users worldwide make audio-only calls.