WhatsApp is currently the biggest instant messaging service in the world, with 700 million users as of January this year. In October 2014, Facebook finalised a $22 billion acquisition of the service, saying that it had no plans to change the app any time in the near future.
WhatsApp had previously announced the possibility of offering voice calls at Mobile World Congress 2014, with co-founder and chief executive Jan Koum initially positing a Q2 2014 launch for the new features.
The current rollout — which has not yet been officially announced by WhatsApp — appears to be operating via an invite-only scenario. According to Patil, the feature gets switched on when a user who already has it enabled calls another user.
Voice-calling will put WhatsApp in direct competition with the likes of VoIP services Skype and Viber. The business model currently has the app free for the first year, then $1 a year after that. Facebook CEO Mark Zuckerberg has said that he was unsure how the company plans to monetise WhatsApp in the short term.
WhatsApp has been contacted for comment and we will update this story when we receive a reply.