whatsapp audio call is unavailable . The reason of that, you are not connected to the data services. when you connect your phone to the internet by wifi or phone data you could make audio calls. In addition, may you have an issue with outdated version of whatapp for that you want to update your whatsapp to the newest version. Moreover, it may be another reasons:
In case you live in a country where WhatsApp Calling is available,then if people who you are calling them live in countries where whatsapp is unavailable, then you will not able to call them.
Another reason could mean that the possessor of the phone has never install and configured the Voice Mail account for his phone.