SAN FRANCISCO: Knock, knock, Google's video chatting app has arrived.

The app, dubbed Duo, represents Google's response to other popular video calling options, including Apple's FaceTime, Microsoft's Skype and Facebook's Messenger app.

Duo isn't much different from the other video chatting services, except that it gives a glimpse at who is making the call, helping the recipient decide whether to answer. Google calls this feature, "Knock, knock".

The new app, announced in May, is being released Tuesday as a free service for phones running on Google's Android operating system as well as Apple's iPhones.

Like FaceTime for iPhones, Duo only requires a person's phone number to connect. Many other services require both participants to have account logins to use their video calling options.

Google has been offering video calling through its Hangout feature for several years, but the internet company is now tailoring that service for business meetings.

Duo is being billed as a simpler, more reliable way to see friends and family as you talk to them.

It is the first of two new mobile apps that Google, owned by Alphabet Inc., has planned for this summer.

The Mountain View, California, company also is preparing to unveil a new messaging app called Allo featuring a robotic assistant that will suggest automated responses to texts.