The new function, which became available for iOS and Android versions of the app, lets users send up to one-minute voice messages to online users, or as voicemails to offline users which can be retrieved after logging on.

To record a message, users press a ‘record voice’ button at the bottom of the screen while in a conversation window with another user before recording their message.

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Facebook has also confirmed it is currently trialling a VoIP service on its messaging app for users in Canada.

According to a news report from Canada.com, users can call other users with the messaging application by tapping the ‘i’ button at the top right hand corner of the application before tapping "free call" to make a voice call. This allows users to make calls using their data plan rather than call minutes, the report said.

"Over the next few weeks, we’re testing the ability to call other Messenger users for free from the app," said Facebook in an emailed statement. "This test will be carried out in Canada on iOS, we have nothing currently to share about if or when it will become more widely available."

According to a description on Facebook’s Messenger app page on the UK App store online store, the company says that the ability to call other users will be rolled out "over the next few weeks".

While alternatives such as RIM’s BlackBerry Messaging service, Apple’s Facetime and Skype already offer free VoIP services, none have the one billion user base currently held by Facebook.

In December, Juniper Research predicted mobile VoIP will reach 1 billion users by 2017, or one in seven mobile subscribers.