Unlocking a phone in Siem Reap

You could try one of these:
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https://www.facebook.com/iphoneservicekh
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http://www.travelfish.org/blogs/cambodia/2012/01/27/where-can-i-get-my-cameracomputergadget-fixed-in-siem-reap/
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http://service.angkortech.net/detail.aspx?services=6&servicesshow=computer%20repair.html
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BTW when they come from manufacturers, iphones usually just have 0000 to unlock them till the new owner puts in their own code.

There are a plethora of shops selling phones (all models /brands) and most will be able to unlock for a couple of US $).