TORONTO » BlackBerry users will be able to make free voice calls over a Wi-Fi network using the popular BBM messaging service.

Research In Motion Ltd. announced today that it’s adding the feature to BBM. Users will be able to switch back and forth from a text chat to a voice call. A split-screen option will let them talk and text at the same time.

The new feature is a free update for existing customers and comes months before RIM introduces its new BlackBerry 10 smartphones, which are seen critical to RIM’s survival.

RIM surprised analysts in September when it announced that the number of BlackBerry subscribers grew, thanks in part to emerging markets and its popular BBM service. It’s struggling in North America as customers migrate to flashier iPhones and Android phone.

RIM stopped short of offering the BBM voice feature over wireless carriers’ own cellular networks. Doing so would have potentially created more congestion on cellular data networks and deprive carriers of revenue for voice calls. With the new feature, the free calls are limited to times and places where Wi-Fi is available.

The Canadian company said the BBM voice feature is especially attractive for developing markets. Unlike regular texts, BBM messages are not charged on a per-text basis.

Although RIM is struggling in North America, the BlackBerry continues to sell well in such markets as South Africa, Nigeria and Indonesia.

The BBM service has long been a reason for BlackBerry users to not defect to other smartphones but there are rival messaging services. There are more than 60 million BBM users worldwide.

RIM said the BBM voice update is currently available for BlackBerry smartphones running the BlackBerry 6 operating system or higher, with plans for BlackBerry 5 later. RIM’s latest phones run the 7 operating system. The next version, BlackBerry 10, will come soon after a Jan. 30 launch event.