You may be heard of the WhatsApp calling feature which allows you to call and talk other users using WhatsApp. This feature was not available for every users, WhatsApp was rolling it out slowly. Now every one can get this feature.
If your WhatsApp still don’t have this feature you get this by updating the WhatsApp from the play store. If you could not find an update on play store, just go to the APK mirror here , download the APK (version 2.11.561) and install it that’s all. (Before install force stop the WhatsApp and clear the WhatsApp cache, added how to at the bottom)You may enable “Install from unknown sources” to install the APK. After that you need to get a call from a user who is already got this feature. If you could not find one, leave your contact number, I can make call.
Make sure you are using version 2.11.561 by checking Settings => Help=> About
Thank you Jayaraj for the call 
To force stop and clear cache
Open Application Manager or Apps from settings
select “ALL”
scroll down and find whatsapp and tap on it
Tap on “Force Close” and then “CLEAR CACHE”