Working at AU you have to be aware of the rules for use of employee phone at MPE:
Download the Solemn declaration about work phones here.
The general policy of MPE is that roaming abroad should be used as little as possible and that you should connect to a wireless network whenever possible. Furthermore, employees are encouraged to use Skype or other internet-based telephone services to call abroad. It is not allowed to make personal calls abroad with your employee phone, regardless of whether you have chosen to be registered with the multimedia tax or not.
You can find more information about mobile telephony at AU here.