[inquisitor]

@alect
AT&T runs GSM and UMTS at 850 and 1900 MHz, so your HTC Desire is definitely AT&T compatible. Since it supports GSM at 850 and 1900 MHz it will work on their 2G network and as you obviously own the version sold by Telstra (Model A8183), which also supports UMTS at 850 MHz it will even work on most of AT&T's 3G network. Only those 3G cells running at 1900 MHz will be inaccessible to your phone, but these places will surely be covered by 2G. So the only effect of the lacking UMTS1900-support is that at some places you won't get 3G and thus no broadband speed. But that's not an issue for voice calls at all.