Whatsapp started rolling out voice calling for all its users last month. Those who were first in line were those with Android smartphones. Now the iPhone crowd can join in as well. At least not now but soon.
Whatsapp is currently rolling out the feature to select users worldwide and will continue adding more and more users as time goes. The Whatsapp application has been updated on iTunes but simply having that update doesn’t guarantee a user access to the voice calling. This is likely akin to how Whatsapp did the rollout on Android.