“Our year-to-date iPhone sales to switchers are the greatest we’ve seen in any nine-month period,” Mr. Cook said in the call, without disclosing precise figures.

An Apple spokeswoman declined to comment beyond Mr. Cook’s comments.

I decided to test Duo to see how the new app plays into the debate about whether iPhone users should consider switching to Android devices. After trying an early copy of Duo and comparing it side by side with FaceTime, I found that Duo was just as intuitive and fun to use as FaceTime, but that it was too early to tell which service was more reliable.

A Short History

First, a little context about Duo and where it fits into the universe of Google apps. The Silicon Valley company announced in May at its annual developer conference that Duo would be coming. The app is solely dedicated to video chats between two people, the company said, to make video calling simpler to access.

This is not the first time Google has provided video calling. Google started offering mobile video calling several years ago inside its Hangouts app, which also includes messaging and voice-calling capabilities. The video-calling feature in Hangouts will remain intact.

Crystal Dahlen, a Google spokeswoman, said the name Duo conveyed that the app was designed for one-to-one video calling on mobile devices, in contrast to Hangouts, which lets groups of people make video calls over mobile devices and computers. Duo will roll out globally over the next few days.

FaceTime, in contrast, is now six years old. When Apple unveiled the video-calling ability in iPhones in 2010, Steven P. Jobs, Apple’s co-founder, said he had dreamed of mobile video calling since he grew up watching “The Jetsons” and “Star Trek.” Apple later expanded FaceTime to work with iPads and Macs, among other Apple products. The company has not published statistics on FaceTime use, but the technology helped make mobile video calling mainstream.

Duo Versus FaceTime

The Duo app is downloadable and free through Google’s Play store and Apple’s App Store. When you open the Duo app, there is just one button: video call. Tapping that loads your contacts list, and from there you can choose whom to call.