WhatsApp has purposely kept its messaging client focused on core features since launching in 2009. While some of its rivals have found success in adding stuff like stickers and games, a note taped on CEO Jan Koum's desk that reads "no ads, no games, no gimmicks" serves as a daily reminder of their commitment to stay focused on building a pure messaging experience. There's one area where they do intend to catch up soon, though.

Speaking at the Mobile World Congress in Barcelona, Koum announced that WhatsApp will expand to voice calls in the second quarter of 2014. The new feature is expected to roll out on Google's Android and Apple's iOS operating systems first, eventually making its way to other platforms like Windows Phone and Blackberry.

Although rivals including Viber, Line, BBM and other have been offering free voice calls for a while, the company feels confident their solution will be the best out there. "We use the least amount of bandwidth and optimize the hell out of it," Koum said. WhatsApp not long ago added "hold-to-talk" voice messages in its mobile clients and it's admittedly one of the most straightforward implementations so far.

With 450 million users worldwide and engagement rates at over 70%, the move is bound to make WhatsApp more of a threat to some mobile network operators, which have already seen revenue from SMS go down.

Koum also took the opportunity to reassure everyone in attendance that the company intends to remain autonomous and operate independently of Facebook following its $19 billion acquisition. "There are no planned changes. We can only get our product to a billion or 2 billion users if we continue down the path we started on."

WhatsApp is free to use for the first year and then $1 per year. The company has vowed never to rely on advertising for revenue and says that as a product they want to know as little as possible about users.