TORONTO -- Facebook has chosen Canadian users to be guinea pigs for a new mobile feature to make free phone calls.

Facebook's new Messenger app for Apple mobile devices enables voice-over-Internet protocol phone calls, which use data instead of eating into the minutes in a mobile plan.

The feature, which allows Facebook users to call each other for free, is being tested in Canada first.

The new version of the app also allows users to send short voice clips to one another rather than typing out a message.

The app, which requires iOS 4.3 or higher, works on the iPhone, iPod touch or iPad.