Research In Motion has come up with a fairly stark method of trying to retain and even win customers: free calls on BlackBerry handsets. The feature does have some significant limitations, but could be particularly popular in some countries.

The free calls will be through VOIP (Voice Over Internet Protocol) and will only be available when you are connected to a Wi-Fi network. The call will run via the BBM messaging service, so you can only speak to your BBM contacts and both parties have to be on Wi-Fi.

To make things simpler, there’ll be a new indicator on your BBM status lists to show somebody can take a voice call. You’ll be able to switch instantly between talking and text messaging or even run both at the same time with a split-screen.

There doesn’t seem any technical reason why the system couldn’t work over cellular data connections, so it seems RIM has decided its not worth the risk of upsetting network providers by “stealing” voice call revenue away. That might be a smart idea, particularly in markets where it relies on networks subsidizing handset costs.

That said, offering BBM voice chats appears to be particularly aimed at developing market countries where users don’t have the same access to affordable call rates or unlimited voice deals as in places like the US.

The feature isn’t being held back for the next generation of BlackBerry devices, due early next year. Instead it’s already available in a free update for the two latest RIM operating systems, BlackBerry 6 and 7. A BlackBerry 5 update is in the works. You should note that the current update is only for a beta version of the feature, so it may not work perfectly at first.