Hi Dennis.
Thank you for your inquiry.
While in Australia, even though you are using your US mobile phone, do not dial 011 when placing calls. 011 is only used to place international calls when dialing from the US. Once in Australia (or any country) your US phone becomes a local phone for outbound calls. So just dial Australian numbers the way locals do.
Now the phone number you have been dialing (1 300 …) is a special non geographic type of phone number used mostly by businesses. Kind of like a US toll free number. These 13x… numbers comprise 10 digits and often can only be dialed from within Australia. The correct way to dial is 1 300 1XXXXX (no extra codes are needed).
I am not 100% certain, but there is a possibility that this number can not be called using your US phone. This might explain why you haven’t been able to reach this number.
All other regular Australian mobile and landline phone numbers, when dialed from Australia, use the following sequence:
0xxx xxx xxx. Mobile numbers start with 4: 04xx xxx xxx. This is what you should dial.
More info below:
https://www.howtocallabroad.com/results.php?callfrom=australia&callto=australia
I hope this helps.