Need Lg Cu500 Help

I have an LG CU500 phone purchased from Best Buy in the USA that carries the Cingular logo with my ATT service provider. The phone is claimed to be tri-band. I am currently working in South Australia and brought my phone with me. I requested my phone be unlocked before I got here so that I could buy a prepaid sim card and make calls through a local network. It was unlocked successfully. However when I tried using prepaid sim cards from Telstra and Vodafone, the two main providers here, everything appears to be normal (my contacts and settings are intact) except for I cannot make or receive calls. Any attempts prompt me with an error that says "Service Unavailable". The frequency here appears to be 900mhz for all major providers. I changed my network settings from automatic to manual and I select the only network that shows up which is "Telstra 3G". I can only use my phone while roaming on my Cingular/ATT network (through Telstra) with my Cingular/ATT sim card. But I want to be able to use a prepaid option because roaming is outrageously expensive. Is there a way to change the frequency settings on this phone? Am I missing another important step? I even spent an hour on the phone with my customer service and they could not help me. If anyone has any insight to this issue I would love to know.

I was able to finally get my phone working by entering a code that brings up an otherwise unavailable service menu. I found the code on another forum while searching for answers. The code is 277634#*#. Seems like my phone's carrier and manufacturer secretly hide this information from the public and I am not surprised. I was able to tinker with settings in this menu in which I changed the frequency from "American" to "European". Seems as if they have the same frequency in Australia as in Europe. What a nightmare it is to try to get your phone to work while traveling!