Toronto - BlackBerry maker Research In Motion is announcing a new feature for its popular BBM messaging service. It will allow its customers to make free voice calls over a Wi-Fi network.RIM said on Wednesday that BBM users will be able to switch back and forth from a text chat to a voice call. RIM also says a split-screen option will let them talk and text at the same time.The Canadian company says the BBM voice feature is especially attractive for developing markets. Unlike regular texts, BBM messages are not charged on a per-text basis.RIM is struggling in North America to hold onto customers who are abandoning BlackBerrys for flashier iPhones and Android phone. But the BlackBerry continues to sell well in markets like South Africa, Nigeria and Indonesia.