WhatsApp will add voice calling later this year, putting even more pressure on big telecommunications companies, after Facebook recently acquired the insurgent company for $19 billion.

What's become the app of the month (at least), WhatsApp CEO Jan Koum told an audience at the Mobile World Congress in Barcelona, Spain this week that the messaging service will roll out free internet voice calling soon. After being acquired by Facebook for a record $19 billion, WhatsApp is already putting the pressure on big telecoms like AT&T and Verizon with free international texting. Now, the move to add virtually free internet voice calling is just piling on.

WhatsApp Messages And Telecom's Texting Fees

While it's certainly not the first or only messaging app to allow users to make virtually free internet-based international text messaging available to its users, Facebook's acquisition of WhatsApp -- the most popular messaging service -- is certainly making telecoms nervous.

It should.

WhatsApp, according to Facebook's CEO Mark Zuckerberg, is on track to have a billion users (currently it has about 450 million). Even taking a conservative view, adding some of Facebook's current 1 Billion+ user base to WhatsApp's own quickly-expanding network entails more than a billion people using an unlimited international texting service that costs users a whopping $1 per year, plus the data they use to send messages.

In what can't be deemed a coincidence, AT&T announced on Tuesday that subscribers to the Mobile Share plan (which already included unlimited domestic text messaging) would get international texting to 190 countries for free as well. This includes many Latin American countries. Verizon's recent "More Everything" announcement includes the same feature. But those free international texting features are only for a certain range of contract, bundled phone plans -- leaving many who prefer prepaid wireless in the lurch. For them, WhatsApp is still the best alternative, but one thing is clear: internet messaging is putting the pressure on wireless carriers.

Voice Calling on WhatsApp

WhatsApp is set to put even more pressure on wireless companies -- especially in the all-important realm of international calling (re: Latin America) -- by adding free voice calls via the internet to the list of core features on the app.

"We are going to introduce voice in WhatsApp in the second quarter of this year," said Koum at the Mobile World Congress in Barcelona, Spain, according to ABC News. "I think we have the best voice product out there. We use the least amount of bandwidth."

All signs point to no price hike from the $1 per annum standard, and while it will undoubtedly cost users more data for the voice function, WhatsApp is clearly aware and looking to minimize the damage (and if people with broadband call their folks from home over WiFi, the data cost is negligible). Currently, WhatsApp allows voice messaging, but the full-on phone call feature will be added sometime in the second quarter of the year for Android and iPhone, and later on for BlackBerry and Windows Phone users.

Again, WhatsApp isn't the first messaging service to allow voice calling over the internet (VoIP) -- in fact, Facebook's own Messenger app added a WiFi calling function about a year ago -- but combine the behemoth Facebook, WhatsApp, and free international calling, and you've got a real threat to the expensive status quo.

For more on WhatsApp, including a forthcoming series on how Latinos use the messaging service and which ones are the best for keeping in touch with friends and family in Latin America, check back with Latin Post Tech, and like us on Facebook or follow us on Twitter.