Like anything in an open market, competition allows for changes to be made rapidly in a market space. Recently many chat applications have been adding VOIP calling as a standard feature. Apps like Google Hangouts, WeChat, and Viber allow, mostly, free calls to users over the internet. Recently the team at WhatsApp has entered the fray. According to Reddit user Pradnesh07, this feature was enabled on his device by someone else calling him. Click here to check out a video of screenshots he took of VOIP enabled in WhatsApp. In short, a user with the VOIP feature enabled called Pradnesh07 lighting it up for him. Reportedly, this feature will work on any Android device with WhatsApp build number 2.11.508. Right now, due to its lack of presence in Google Play, side-loading the app is the only option for installation.
The video released by Pradnesh07 shows clearly that the app is segmented into three parts: calls, chats, and contacts. All the standard internet phone features are there like speakerphone, instant messaging, and mute. You can even send IMs while in a call. Since this is a huge feature not officially released by the team at WhatsApp, it seem like it has rolled out to a small group of test users.