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I’ve never really understood either why a paid-up-front phone would ever be locked at all, nor how to verify a given phone is unlocked without having to call anyone.
 
I think many people are confusing "unlocked" with "sim-free". Two different things.

The sim-free versions come out 1-2 months after the release. They come without a sim card, which means they are not linked to any particular carrier (via branding or other means). The sim-free iPhones are also fully unlocked.

However, on launch day, you can buy directly from Apple and by paying full price, that phone will come unlocked, regardless of the carrier you choose. So it's essentially the same thing as buying the sim-free version, except that, despite being unlocked, it comes with a sim card for that carrier and is branded for that carrier, if applicable.

Another misconception is that the "sim-free" models sold in the USA are global phones, which is not true. Sim free simply means they'll work with any carrier in the USA. However, historically, they will not work optimally in other countries. Take the iPhone 8, for example. The sim free version is missing crucial LTE bands that some carriers rely on throughout Asia. Your sim-free model will still work there, just not optimally. In order to get a global iPhone 8, you would have to opt for the Sprint model, which is a true "global" version of the iPhone 8 that most other countries got, which will unfortunately not work optimally for some carriers in the USA, such as Verizon.
See I like that when someone knows what they're talking about. Thanks!
 
I’ve never really understood either why a paid-up-front phone would ever be locked at all, nor how to verify a given phone is unlocked without having to call anyone.
Carriers claim it’s to prevent theft - thieves are less likely to steal handsets locked to specific carriers because they’re more difficult to move, particularly overseas to areas where the phones are harder to obtain.

Common sense says it’s to make switching carriers more difficult.
 
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