The WhatsApp Messaging application now allows you to make calls to other contacts on Android. Thus using a VoIP service, it comes compete with other players in the field such as Google Hangouts, Viber, and of course Skype.
Make sure to have updated the application on Android to enjoy, to discover a new "Call" tab. Once on this tab, we find their contact list, and therefore the possibility of moving them a voice call. If these are not available at the time of the call, they will receive a missed call notification.
WhatsApp is Adding Video Calls Soon 2016