Google Duo is a one-to-one video calling application for Android and iOS which is designed to work in fast or slow connections.
"With Duo, our goal is to make video calling faster and more reliable, even on slower network speeds. We also introduced a feature called Knock Knock, which gives you a live video of the other caller before you answer." said Sundar Pichai, CEO, Google.
Just like WhatsApp, Duo works based on the phone number and allows you to find anyone through the contact list.