To avoid this. I usually buy a different carrier's model (if they have the bands I need). This way even if it's blacklisted on the original carrier i would still be able unlock it for another carrier.
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Nokia 3310 > Samsung D415 > Samsung D600 > iPhone 2g > iPhone 3g > iPhone 3gs > Mytouch 4g > Mytouch 4g Slide > HTC Amaze > Galaxy S2 Hercules > d2tmo.