During a keynote at the Mobile World Congress in Barcelona this week, WhatsApp CEO Jan Koum announced that the mobile messaging service would start to offer voice calls. The move could help justify the massive $19 billion price tag of the recent Facebook deal. While the marketplace for Internet phone services is already crowded, WhatsApp could become competitive since it is now backed by social giant Facebook. The voice service is planned to launch in April.

Koum says the voice service will first launch for iPhone and Google Android smartphones, followed by BlackBerry, Nokia, and Microsoft Windows Phones. The biggest competitor to WhatsApp’s voice service will likely be the immensely popular Skype.

According to The Wall Street Journal, “WhatsApp credits its simplicity and lack of advertising with helping it attract 465 million users for its messaging service; WhatsApp charges 99 cents after the first year, while many ad-supported services are free.” Koum said they plan to proceed with a simple approach to the app, with a focus on ease of communication.

“We want to continue to have that minimalistic approach to the product,” Koum said. “We at WhatsApp want to get out of the way and let people communicate.”

Although WhatsApp does not generate much revenue, Facebook’s chief exec Mark Zuckerberg said, “The reality is that there are very few services that reach a billion people in the world. They’re all incredibly valuable, much more valuable” than WhatsApp’s $19 billion price tag.

“The address books, mobile-phone numbers and other user information in WhatsApp could be valuable to Facebook because it could be used to improve ad targeting,” notes the article. “A Facebook spokeswoman said last week that WhatsApp won’t share data with Facebook, but that policy could change in the future.”