Steps Need to Done :
- Update whatsapp to latest version 2.11.561+ now it is available in google play store
- Ask someone to call your number who having whatsapp call feature right now
- After see your interface will changes to three tabs like below image
- You can call now who ever want this feature
Have fun with whatsapp calling. But hike is already implemented their beta calling features before 3 months try that and post about quality of call.
Note : For version 2.12.5 It is not working. they limited the call features.