How to use Google Duo, the video calling app that's better than Apple's FaceTime and works on any phone (GOOGL, GOOG)

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A few months ago, my older sister made a life change that was met with animosity among the rest of my family members: she switched from an iPhone to a Google Pixel 2. 

Now, I'm a huge fan of the Pixel 2. It's my favorite Android phone on the market, and I'm considering making the switch once I pay off my iPhone 6s. 

My problem was that my sister lives nearly 400 miles away from me, and she has two young kids. By switching to an Android phone, she seemingly eliminated...