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)