As @phb notes they can add a “false” number to their call, though most often it is through the use of legitimate Australian numbers bought as blocks from the Australian Government through a VOIP provider. From the wiki article of Caller ID spoofing, which is what @phb is discussing ie Caller ID spoofing, “Companies such as these lease out thousands of phone numbers to anonymous voice-mail providers who, in combination with dubious companies like “Phone Broadcast Club” (who do the actual spoofing), allow phone spam to become an increasingly widespread and pervasive problem”. It is the lack of restriction on using Australian numbers only for Australian businesses and individuals which creates most of the issues.
The Spoofers can cycle through these numbers easily and when you try to ring one back if the VOIP service has it as inactive you will get the disconnected service message. Happens quickly ie the spoofer moves to their next active number in their list of available caller IDs as easily as clicking on the next choice in their system. They may cycle through them for every batch of robocalls that are made and may not come back to that number for days or weeks.