How to unlock SIM card on my iPhone?
How to activate or unlocked sim
[Re-Titled by Moderator]
iPhone 7, iOS 15
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If you mean the SIM card is locked, you need to get the PUK code for it from your carrier.
If you mean your phone is locked to a carrier, you must contact the carrier it is locked to and ask about their unlocking policy→How to unlock your iPhone for use with a different carrier - Apple Support