Not only its carrier-locked, it is sometimes model (variant model) specific so after unlock you are still stuck with that carrier because the other carrier will

Not only its carrier-locked, it is sometimes model (variant model) specific so after unlock you are still stuck with that carrier because the other carrier will not take it. For instance, after 1 year T-Mobile paid service it finally unlock your phone, thinking about moving to AT&T. AT&T and its MVNO Cricket Wireless actively disconnect your service when you put the SIM card to an "unsupported phone" just because it is not on their list.