The app is currently positioned as an affordable option for users to communicate with global contacts via text messaging.  Its services include unlimited text and voice-mail messaging. WhatsApp remains free to users for a full year. After one year, a 99 cent annual fee is billed.

Apple and Android smartphones will be the first devices to enable voice calls. Windows phones and Blackberry's compatibility is set to come later.

"We want to make sure people always have the ability to stay in touch with their friends and loved ones really affordably," WhatsApp CEO and co-founder Jan Koum explained at the Mobile World Congress in Barcelona, Spain on Monday.

Once voice calls are enabled on all smartphone devices, WhatsApp will become a primary competitor with flagship messaging apps like Skype. Mobile carriers like Verizon and Sprint will also face a threat due to WhatsApp's currently unmatched affordability.  

At the moment, WhatsApp boasts 40 million users in India and 38 million users in Brazil. Reports suggest Facebook's recent acquisition of the app is related to its interest in the huge population of newly tech-savvy users found in these countries.

Facebook announced its acquisition of WhatsApp for $19 billion cash last week.

Follow Executive Producer Marisa Okano on Twitter.