Stumbled across this while searching for something else. Telstra now offers "WiFi Calling" for the iPhone, which allows Telstra customers with a compatible iPhone (with the latest Carrier Update which was released in May) to use their WiFi connection to make and receive phone calls. This was rolled out last year for Samsung phones. It is used when you are in a poor phone reception area but have a working internet service and a WiFi network - if your phone detects a poor 3G/4G connection it will make the call over your WiFi and route it through the internet to its destination. I am currently in an area with only one bar (sometimes No Service) on the phone but had no problems with a WiFi call through a Telstra wireless modem connected to a 5m antenna. Don't know if Optus or Vodafone have similar offerings. More details at
https://crowdsupport.telstra.com.au/t5/Announcements/Telstra-Wi-Fi-Calling-FAQ-s/ba-p/619260