A friend has a Vodafone provided Nexus S. He recently switched to Telecom and now says he has no service in Tawa. I can imagine he has no 3G service if in Tawa if Telecom only has 850 and the phone only does 900/1700/2100 but he should have voice service.
I should check but I wonder if he has no 3G icon on the phone and think it means there is no service at all?
System One: Popcorn Hour A200, PS3 (US 60G) dead and now replaced with a PS3 SuperSlim, NPVR running on Sempron 3000 (XP), Sony BDP-S390 BD player, Logitech Revue, Pioneer AVR, JVC 56" D-ILA 720P RP TV
System Two: Popcorn Hour A110 , Oppo BDP-80 BluRay Player with hardware mode to be region free, Vivitek HD1080P 1080P DLP projector with 100" screen. Harman Kardon HK AVR 254 7.1 receiver, Toshiba HD-A2 HD-DVD player, Roku XS media player
If you don't have an 850MHz 3G capable phone, you will have very little coverage in New Zealand. If the phone doesn't have it, its simply not suitable for XT, unless you know where the 2100MHz coverage is, and that suits you (hint, its only in parts of some cities, so its unlikely to suit many people at all).
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