Blackberry has become the latest smartphone to offer free wi-fi calls to users via its own software. Research In Motion (RIM) has added the facility to its Blackberry Messenger (BBM) app, which already offered an alternative to text messages.

BBM Voice will only work if both caller and recipient are using the program.

BBM has previously been able to send voice notes. And other Blackberry apps - such as TringMe and MobileVoip - already offer access to free calls. However, users will now be able to use BBM's popular text tool at the same time as they are having a free phone conversation.

While the move helps bring RIM closer in line with its rivals, it still lacks the video chat functionality and 3G-support found in Apple's Facetime, Microsoft's Skype and Google+ Hangouts.

BBM Voice only currently works on Blackberry 6 and 7 devices, but RIM plans to extend this to Blackberry 5 handsets at a later point.

Now the company's share of global mobile phone sales had fallen to 2.1% in the July-to-September quarter. Samsung was in the lead with 22.9%, Nokia second with 19.2%, and Apple third with 5.5%.