Australia nextg orbit artemis wizard mini s roaming

hello
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On yet another visit to Australia I thought I hade cracked it ,with cdma being switched off at the end of January and all my friends having nextg phones on 850mhz I thought my o2 orbit or the mini s (both uk 850mhz phones) would work but no luck They did roam in the city but not in the remote parts
if I put my uk o2 sim in one of their nextg phones my number did work at the remote locations
So has any one got any ideas what I need to do to get a fully roaming phone


thanks

John