At its I/O conference back in May, one of the products Google unveiled was its cross-platform video call app, Duo. Today, the company finally launched the FaceTime rival, which is designed solely for smartphones.

The app is available on both Android and iOS, and unlike many of the company’s other products, you don’t require a Google account to use it. Duo works in the same way as WhatsApp: users register and find other people via their phone number.

Google is pushing Duo’s simplicity as its main selling point. It doesn’t have the kind of features found in Skype or Hangouts, such as conference calls, and the company believes this will appeal to consumers.

“Users are reluctant to video call because they don’t know if the other person is on the right network, the right device or it’s a good time to call. We’ve tried to remove all that friction and make it feel like an invitation, not an interruption when someone call’s you,” Amit Fulay, Google’s group manager for communications, told The Guardian.

“Duo is all about simplicity and quality. It’s all about video calling. There are no frills, no knobs or dials to adjust, it just works.”

Duo does, however, come with a unique feature called “Knock Knock.” When a person receives a call, they’ll see a full-screen live video feed of the caller even before they answer. The person making the call gets a notice informing them that “your video is visible,” so they know not to pick their nose or anything while waiting for someone to answer.

The feature may sound a bit worrying, but you only receive Knock Knocks from people in your contact list. It can be disabled completely, and on the iPhone it only works if you’re in the app, otherwise you just get a notification when someone calls.

Duo fully encrypts all calls and adjusts the video resolution automatically depending on the speed of your connection. It can even keep a call going should you decide to switch from Wi-Fi to cellular. You can register to download here.

While some have praised Duo’s simplicity and clean interface, there are those who say it’s these elements that make it dull. The video calling market is a crowded one, but a combination of Knock Knock and not having to create an account could help Duo stick out from the rest of the pack.

Google's app may not replace FaceTime for iPhone users, but it does have potential on Android – early user reviews are overwhelmingly positive.

Duo is rolling out worldwide on the Play Store and App Store over the next few days.