Facebook is preparing to launch a new feature for its Messenger app which allows users of the social networking site to place free voice calls to friends.

The feature is so far available only to smartphone users in Canada and is buried within the latest update to the app, but it will eventually allow users to make free internet voice calls, known as VoIP calls, to any Facebook friend.

Experts are saying it represents an attempt by the world's largest social network to dominate the social world by taking on the default calling function in mobile phones, the 'Daily Mail' reported.

The new feature comes at the same time as Facebook Messenger rolled out a new feature worldwide which allows users to record and send a voicemail-type message to friends.

Working in a similar way to video messaging in the company's Poke app, users press and hold a red record button, speak their message, and it appears in line as part of the conversation.

TechCrunch writer Josh Constine imagines a range of uses for the function, from messaging while driving to recording the waves lapping at a beach to send to friends.

However, its addition to the Messenger app seems merely to make it an 'even more complete app' he writes, adding that he expects video messaging to soon be added as well.

One-tenth the size of the US, but with very similar demographics and mobile usage trends, Facebook is using Canada as a testing ground in advance of rolling out the feature in other markets, the paper said.

By clicking the 'i' icon in the top right of a conversation in the most recent update to Messenger, users reveal a 'free call' button which allows them to contact any friend also within the test region.

However, while Facebook is not charging users for the service, the call is not technically free since it will use data on users' existing mobile plans.

TechCrunch said that the move into voice messaging and VoIP can be seen as an attempt by the social network to take on the default, mobile network operated calling function on smartphones.