The messaging app has officially added voice calling, a long-teased feature that was previously available only by invitation.

WhatsApp users can now call their contacts worldwide for free over a WiFi or 3G/4G connection.

After making a name for itself with free text messaging, WhatsApp is breaking into the VoIP market with its latest feature, introduced in the latest update for Android (2.12.19) and soon to come to iOS. A new Calls tab allows users to manage outgoing and incoming calls, and to call any online contact simply by clicking on their name. Contacts who do not respond receive a missed call alert.

The move places WhatsApp in direct competition with Microsoft's Skype and Rakuten's Viber, both of which allow users to make voice calls between online contacts and -- for a fee -- to any number, landline or mobile. Given the massive scale of WhatsApp's user base (over 500 million worldwide), the app is now also a significant alternative to mobile operators' pay-as-you-go plans. In markets where plans with unlimited local calls have become the norm, the new feature could still be of interest as a way to stay in touch with contacts abroad, whether over WiFi or data networks.

WhatsApp users have already had the ability to record and send voice messages to their contacts since 2013.

WhatsApp, a subsidiary of Facebook, is available for Android and iOS as well as in a web version introduced at the start of this year.

The service costs USD 0.99 (after one year free) and allows users to send and receive text messages, photos and videos from any location.