Google is to roll out its Duo video calling app worldwide over the next few days.

The app is a free service for phones running on Google's Android operating system as well as Apple's iPhones.

It only requires a phone number, and not an account login, meaning it will only work on smartphones – not tablets or computers.

It is Google’s answer to Apple's FaceTime, Microsoft's Skype and Facebook's Messenger.

All calls will be encrypted and users will be able to see a video of the incoming caller before answering, which the company is terming its ‘knock, knock’ feature.

“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,” said Amit Fulay, Google’s group manager for communications.

“We’ve tried to remove all that friction and make it feel like an invitation, not an interruption, when someone calls you.

“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.

“The chances of you having your close friends and family’s phone number is much higher than having a specific account, so using the phone number makes it easier.”

Duo will look to capitalise on the fact that iPhone users can only use FaceTime with other users of Apple products, although that software does work on iPads and Macs.

iPhones have a minority share of the smartphone market, where Android is dominant.

The existing video conferencing technology Google Hangouts is now being targeted at business meetings.

Google, which recently smashed expectations with its financial results thanks largely to video and mobile, is also preparing a new messaging app called Allo.

It will feature a robotic assistant which suggests responses to texts and comments on friends’ pictures using image recognition technology. 

Google has big plans for artificial intelligence through British start-up DeepMind, which it bought in 2014.