WhatsApp CEO Jan Koum has no plans to make money out of the billion-dollar messaging app bought by Facebook, as yet.

 WhatsApp, the popular cross-platform instant messaging app, will focus on product development rather than on revenue generation for the next few years. Jan Koum, the CEO of WhatsApp, said the company will not explore monetization experiments with its users after a failed attempt before the acquisition. Instead, they will invest to grow their reach to 1 billion users, which is a projected goal for Facebook CEO Mark Zuckerberg, to start rolling out ads.

WhatsApp previously tried charging users in some countries a fixed subscription fee of $0.99 to keep the service running without ads. But the response wasn't great so the model was shut down soon after. Facebook found the app's growth interesting and bought the company for $19 billion in February and appointed Koum as a board member. WhatsApp currently has more than 600 million users and Facebook's parenting makes it a good fit for advertisers to promote their products.

"For a bit, we can focus on growth only and not have to do any kind of experimentation with monetization," Koum told Re/code's Liz Gannes at the Code/Mobile conference,Tuesday.

Koum took the opportunity to discuss the immediate growth plans for WhatsApp, including the addition of voice call technology as early as first quarter of next year. The app developers have constantly updated WhatsApp for better chat experience. The news of adding voice calls originally came to light in February, when Koum spoke of the company's plans at the Mobile World Congress in Barcelona. By adding the new feature, WhatsApp will compete against services such as KakaoTalk, Line, BBM and other messaging apps with similar offering.

While it is expected that the app will continue to add more features, it is certain for now that there won't be ads or subscription fees to use the service for the next few years. Zuckerberg also shares a similar vision for WhatsApp.

"Products aren't really interesting as a business until they have a billion people using them," he said during Facebook's Q3 earnings call, Tuesday.