Step 1: Download latest version of WhatsApp for Android (2.11.561) from the Play Store
Step 2: Ask a user who has the feature to call you..
Step 3: After receiving your call, close the app and reopen it.
Step 4: Check out the new tab for calls. From here, you can call your buddies over VoIP, and the more people you call, the more it will spread.
You can then call any of your WhatsApp contacts over VoIP through the app. You may not be able to reach people running older versions of the app