Telstra is primarily using GSM 900 in Australia as they were able to get large bands of spectrum early on. The other carriers have GSM1800 fill in many areas even though their primary license is GSM900. Hence you will get GSM1800 coverage on Optus and Vodafone but it may NOT be the best.

Bottom line is that you are getting coverage only on GSM1800 - which will be spotty compared to GSM900+GSM1800 dual network coverage.

If you do travel outside the US often I would highly suggest seeing about getting a quad band enabled BB like the 7100g or the 7290