The instant messaging app imo has crossed the threshold of a half-billion Play Store installs, and there's a pretty good chance you've never heard of it. But with the rise of challengers like WhatsApp ...

While Apple's FaceTime popularized video calls on smartphones, there are plenty of other apps that now offer a top-notch experience. Shelby Brown (she/her/hers) is an editor for CNET's services team.