Optus has launched an app for iPhones and Android handsets that allow voice calls and text messaging over Wi-Fi connections.
The WiFi Talk app is available for Optus mobile customers with handsets running Android 4.0 or iOS 6 or newer versions of the mobile operating systems.
Optus customers will be charged the same rate as they would be over a cellular connection and for recipients there is no visible difference from regular calls and SMS messages.
Vodafone has previously examined support for Wi-Fi calling. Earlier this year Telstra indicated voice over Wi-Fi was part of its mobile roadmap.
"Optus is focused on giving our customers the best possible experience," Optus vice president of customer experience and delivery, Amanda Hutton, said in a statement released today by the telco.
"WiFi Talk is an innovative solution to help customers stay connected easily if mobile coverage is limited indoors, whether at home, in the office, or even in places such as shopping centres."
The release of the app is the "first of many exciting developments for Optus in the WiFi calling space," Hutton said.