We're not allowed to discuss it in detail in the forums, but there are other ways to make a phone appear as if it's unlocked, but it actually isn't. Then when you try to use it on some other carrier or some software update is downloaded, it reactivates the carrier lock. As often as I see threads in these forums about this problem, I was wondering if it is only Verizon phones which have the problem, but no...I went and looked at another large US carrier's forums just yesterday and a regular user at the other place gave more details about how your problem happens. Their forum had an entire section dedicated to locked device problems and it was full of threads exactly like yours.
In a nutshell, the best way to buy an unlocked phone is directly from the manufacturer. There is no telling what you'll get otherwise.
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I'm not a Verizon employee, just another customer trying to help.
