The world's biggest messaging service WhatsApp, which Facebook bought for $19 billion recently, will add voice calls to its product in the second quarter of this year, its chief executive Jan Koum said.

With 450 million users worldwide, WhatsApp and its competitors South Korea's KakaoTalk and China's WeChat have punched a hole in telecom operators' revenue in recent years by offering a free alternative to text messaging.

The news that the most powerful of them was adding voice calls to its service will likely be seen as worrying for telecom operators globally, which got about $120 billion from text messaging last year, according to market researcher Ovum.

"We are driven by the mission that people should be able to stay in touch anywhere and affordably. Our goal is to be on every mobile phone in the world," Koum said on Monday, speaking at the Mobile World Congress in Barcelona.

Koum said WhatsApp's acquisition by Facebook would not alter his roadmap to develop the product to reach the next 1 billion users. No advertising will be added to the service, he said.

Facebook's chief executive Mark Zuckerberg was also slated to speak at the event.

WhatsApp's Koum also sought to cast himself as a partner to telecom operators and not simply a competitor. He announced a partnership with KPN's E-Plus under which it will launch a WhatsApp branded mobile service in Germany.

"We are working with carriers in established markets to bring value to end users," he said. - Reuters