How to unlocked your phone by the service provider
The moment the sim in entered I get this sim not supported and it should be unlocked by the service provider notification. How do I unlock it? And who’s the service provider ?
iPhone 8 Plus, iOS 15
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Only the carrier it's locked to can unlock it.
How to unlock your iPhone for use with a different carrier - Apple Support
If you don't know what carrier it's locked to, ask whoever you got the phone from.