Voicemail and Call Back are two new features offered on the latest WhatsApp beta
Call Back button is second from left, Voicecall is third from left
The new features become options that a caller can select once a phone call is not answered, or is declined. With Call Back, you are merely trying to redial the call you just made. With Voicecall, you press the mic icon to leave a voice message right from the same screen used to dial the call. One of the most popular messaging apps on the planet, there is talk that video calling will soon be coming to WhatsApp.
You can wait for the new features to make it to the App Store or Google Play Store, or you can download the APK. To do that, click on the sourcelink.
source: PhoneRadar (1), (2)