When you are expected to return business related calls from your mobile phone and you wish to not share your personal (australian) mobile number for whatever reason, you can do the following:

A. Block your number on a call by call basis 

From a landline, dial 1831 then the number you are calling

From a mobile phone, dial #31# then the number you are calling

B. Block your number permanently 

For mobile phones, most phones include a “show my caller ID” setting in the settings app, which can be turned off.

On an iPhone go to Settings > Phone > Show My Caller ID

On Android phones to Phone > Settings > Call Settings > More Settings > Caller ID (or similar)