Whats App, being one of the highest used app on almost every smartphone that has an internet connection on it.
So why not use a single app, by which one can chat, share and talk, using the internet.
There are random times in a day or week when the Whats App servers are online, and without any notice.
So if you are at the right time to get a call, you are lucky.
If the contact you are calling does not have the updated version, it will not be listed in the contacts for voice calls.
Now here is a trick to get Whats App calling enabled—but could be risky.
However, the person you are calling has to also have the updated version of the Whats App app.
They need to assure that the data used in voice calls are minimal and secure.