Since last friday, many whatsapp users are enjoying voice calling feature, Whatsapp opened the invitation for one full day and may users are now enjoying the service. But currently the invitation server are closed again and it may opened in next few days.
However rooted device can till change the file ‘com.whatsapp_preferences.xml’ in data/data/com.whatsapp folder by adding the following code
<boolean name="call" value="true" />
And force closing the app will enable the calling interface, but the outgoing call feature will not be available, however incoming call will be available.
May be the call activation server will open soon and the calling feature will get activated then.