Jisher wrote:

Is it possible to STOP outlook from forcing mobile numbers to show
the area code, eg: (04) 0000-0000?

No. The masking of phone numbers is hard-coded in Outlook.

I'd like to keep the area code for other phone numbers, however would
like to stop it happening for mobile numbers?

There should be no need. Any current phone should be able to handle area
codes.
--
Brian Tillman [MVP-Outlook]