Duo could be used either with a phone number or a Google account, allowing users to call someone from their contact list.

[10] Google Duo was optimized for low-bandwidth mobile networks through WebRTC and uses QUIC over UDP.

[12] In February 2021, Google announced a new very low-bitrate codec for speech compression called "Lyra" that could operate with network speeds as low as 3 kbps that avoided robotic voice audio and that was to be rolled out to Duo.

[13] According to a technical study commissioned by Google from the Signals Research Group in 2017 that compared degradation time over 3G, 4G, 5G and Wi-Fi, Duo provided the highest voice and video quality of any service or app.

[22] In line with similar group calling offerings from FaceTime, Skype, WhatsApp, and Messenger, participants could join or leave the conversation at any time.