Whatsapp Voicemail Feature was recently introduced with few extra features. Voicemail is once such feature. When you call your friend via WhatsApp and he did not respond, we will get three options. They are Cancel, Callback and Voicemail.
All you need to do is just hit the Record Voice Message button and long press it. Record the message you want to convey. Now you are done. The Voicemail is sent now.
Here is the twist, this feature is not available in on your recent version of Whatsapp on your device. This is available only in the latest version "2.16.189". and you can get it from APKmirror website. You can download the latest version with Voicemail Feature from Here