turpe Apple issued automatic software updates a while back that screwed up a bunch of unlocked iPhones here in the US.
They claim that the problems were 'unavoidable', but hairy conspiracy theorists in high-tech log cabins think that coding may have been included for the specific purpose of cutting off those that chose to defy Apple's desire for all US iPhones to be on AT&T.
Just saying.
Buy iPhone - unlock it - do not update the software = job done.
Buy iPhone - unlock it - do not update the software = job done.