Recently a dutch website released some screenshots of this features which they are claiming to be extracted from official website of whatsapp. These images show how this features will function in new whatsapps. These are given below
You can see in the above image which shows the upcoming whatsapp voice call features preview. If they are to be believed then this would give a clue that whatsapp voice call features is to rolled out soon probably next year start.
Also the website told that call function will work like smartphones which is sliding to take calls and bluetooth feature will also be introduced while messaging feature and others will remain same as they were in the past.
Another thing that is important to know is that Play store version of whatsapp is not updated in real time and this feature will be definately be available at official version of whatsapp at its own website.
