WhatsApp, maker of one of the planet’s most buzzed about mobile apps, has always tried to keep a low profile. That’s not nearly as easy as it used to be.

In February 2014 Facebook paid $22 billion for the stealthy mobile messaging start-up. Since then,it has topped 1 billion users, a milestone Facebook has said is the first step toward contributing to the social network’s bottom line.

Popular new features such as voice calls are broadening its appeal, especially overseas. People are now making more than 100 million voice calls each day from the messaging app.

WhatsApp has thrust itself into the international debate over smartphone privacy by introducing full encryption, making it more difficult — if not impossible — for law enforcement or anyone else to gain access to messages sent over the app.

Forcing Jan Koum, the company’s reclusive co-founder and chief executive, even farther out of his shell is his goal for WhatsApp — that everyone with a smartphone use the messaging app.

“We’re nowhere near that,” Koum says of the goal. “But we hope that over a certain period of time we will get that critical mass.”

One of the hurdles to achieving it? The United States, its home base. “Historically it has been a challenging market for us,” Koum says. “There a lot of products that people use here.”

Facebook paid a steep price for a mobile app with negligible revenue that, while widely used internationally, was less known in the United States. Yet its startling growth, faster even than Facebook’s own in its early years, caught the attention of Facebook CEO Mark Zuckerberg.

Zuckerberg, determined to break into the growing mobile messaging market by betting on a promising new player, had twice tried to buy Snapchat. With Facebook’s technical and financial heft, WhatsApp’s audience has doubled since the acquisition

Koum, who sits on Facebook’s board, runs the company largely independent of Facebook, in a building that does not bear the company name, about 10 miles from the social network’s sprawling campus. A self-taught programmer and college dropout, he created the messaging app in 2009 with his one-time Yahoo colleagueBrian Acton. People trade 42 billion messages, 1.6 billion photos and 250 million videos each day on the app.

With the growing ubiquity of WhatsApp, Koum, who for years assiduously avoided the media, has begun to open up, speaking with USA TODAY this week about WhatsApp’s latest voice calling news, as well as the path it has charted to growing its global footprint and making money.

The voice-calling feature launched more than a year ago, plunging WhatsApp into a heated market where it competes with Microsoft’s Skype service.

“We didn’t think (voice calling) would become such a big part of our app,” says Koum, who was born and raised in a small village outside of Kiev, Ukraine and immigrated to the U.S. when he was 16.

“A lot of people were writing in saying, ‘We want to be able to make voice calls,'” Koum says, so Koum and Acton decided in 2013 to build it for them.

There were hiccups. Koum announced voice calls would be coming soon during Barcelona’s Mobile World Congress, in 2014, shortly after the Facebook deal was announced, but the feature wasn’t up and running until 2015.

“It was the first time ever in the history of the company that we pre-announced a feature, and we missed the deadline by a mile. That was embarrassing,” Koum says. “The good part is when it launched it was really good quality.”

WhatsApp’s philosophy: To build products that can work on any device and on any network, no matter how primitive. For voice calling, that amounted to an engineering feat. People use WhatsApp to make voice calls on all kinds of devices, from Apple’s iPhone and smartphones using Google’s Android operating system to Microsoft’s Windows Phone to phones made by Nokia.