This voicemail feature is very similar to Voice messages which we have been using in WhatsApp since long time but what makes me important is coming to the Voice call screen. Now, on whenever you make Voice call on WhatsApp and if the recipient declines or dont pickup the call, unlike before you will now see a new screen with 3 buttons, i.e. Cancel, Callback and Send Voice Message. Simply press and hold the Mic icon to record audio message. This helps you to skip going 2 screens back to make the call again or send text/audio message.
To use this feature you must have subscribed to WhatsApp Beta and must be running the latest Beta version, i.e. WhatsApp v2.16.195. You can download the APK file from here or get it directly from PlayStore.