Transfer data from old iPhone to new iPhone
How do you transfer your phone number and all your data from your old iPhone to your new iPhone? And can you use it as a phone once you transfer all the data onto the new phone?
iPhone 12 Pro Max
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Use Quick Start to transfer data to a new iPhone, iPad, or iPod touch - Apple Support
How the number migrates depends on the carrier and the plan. Might be a SIM swap, might arrive pre-loaded in the eSIM (if purchased that way), might need a call to the carrier to swap the line. The migration (above) will usually tell you about that detail for your carrier and your case.
The old iPhone will work as it has after the migration, though you’ll either need to use only Wi-Fi for data transfers, or can (re-)add a cellular plan if you want that.