It will take some time to roll out in every iOS or iPhone mobile devices. So, don't be fret, if you are not notified for this update.Soon, you will get it for your mobile phone.
This works like someone calls you through Whatsapp, you will be notified with a push notification showing the caller information. Once you you pick the call you will get option to mute or put call in speakerphone. Android phones shows you call log of received, missed Whatsapp calls. Let's see how it will work for iOS exactly.
For more information visit Forbes.