I found this. Does this apply to "unlocked" iPhone bought from Apple?
Is another step (blowing the fuse) necessary to make it permanently unlocked?
I need to be able to use one SIM in the morning and another in the evening, to make it clear what I need.
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https://www.quora.com/How-does-a-carrier-network-lock-work-How-is-it-implemented
It's implemented by the control program running on the baseband processor.
A cell phone is two separate computer systems by itself, the application processor, which runs Android or iOS, and a completely separate baseband processor, which runs the actual software that directly interfaces
with the cellular network. A SIM card is also a complete computer system (it has RAM, ROM, a CPU, and a version of Java; all communications with the SIM is actually with a Java program running on the SIM), but that's not important for this question.
The baseband modem's system software checks the first five or six digits of the IMSI from the SIM against a pre-configured value. These first five or six digits form the Mobile Country Code and Mobile Network Code, which combined identifies the network the SIM is for.
There is a tiny electrical fuse (like the ones in your car's fuse panel, but microscopic) inside the baseband modem processor that is hooked up so that it can be “read” as computer memory. When a permanent SIM
unlock is performed, the baseband software blows the fuse, which cannot be restored. The software checks this fuse when it checks a SIM card at power on, and if it reads as blown, it skips the network ID check and assumes the SIM is good. A fuse is used so that once a phone is unlocked, it cannot be relocked.
