Just yesterday it was reported that WhatsApp was opening up a flow of beta invites to users to enable the long anticipated calling feature through the instant messaging application, and that users who were included in the beta could send out invites to other users in their contacts list simply by calling them. Initiating a call to any contact was stated as the process for sending out the invites to others who did not yet have calling activated. At the time it was unclear how long users would have to be able to invite people from their contacts to enable WhatsApp calling, and was stated that it could be on a little while longer than the first time WhatsApp began these tests.
According to a blog post on GadgetRaid from earlier it appears that beta invites for the WhatsApp calling feature are now closed with users stating that they could not activate the calling even after receiving calls from users who had it enabled. The source mentions that they tested this on one of their own devices that did not have the feature activated, having made calls from another device they had which already had WhatsApp calling enabled, resulting in no possibility to activate the feature on the first device. This seemed to persist even after uninstalling, rebooting, and reinstalling the application, just to verify whether or not it had anything to do with being on the latest update.
If all of this is to be believed, then the window for activating calling within WhatsApp seems to have been shut down by WhatsApp for the time being. It should be noted that users who don't have the calling feature enabled and can't make outgoing calls using WhatsApp should still be able to receive calls from people in their contact list who do have the feature activated. At this point in time, there is no word on the time frame for when more invites for the calling feature will be opened up, or when the feature will officially go live for any users who are on the latest version of the application.