When you migrate from your old iPhone to a new iPhone, the SIM card has no role to play in the actual migration process. The transfer of your apps, settings, photos, messages, accounts, and other data is handled through the iPhone-to-iPhone transfer process, iCloud, or another supported transfer method—not through the SIM card.
You can therefore complete the migration and set up the new iPhone without transferring the physical SIM card first. The SIM becomes relevant after the new iPhone has been fully set up, when you need to activate your cellular service and make or receive regular phone calls and SMS messages.
If you use a physical SIM, you can move it to the new iPhone after the setup is complete. If you use an eSIM, you can transfer or activate the eSIM during or after setup, depending on your carrier's support.
In simple terms: data migration and cellular service are two separate processes. The SIM is not what transfers your data from one iPhone to another; it is primarily what connects the finished iPhone to your mobile network.
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