How do I activate my new iPhone with cellular service

Get a new SIM from your carrier and install it.

For now, you can take the SIM out of your old phone and use that one but if your phone is older than a 12 and your new phone is a 12 or 13, you won't get 5G unless you request a new card from your carrier.

Lila-arzillo Said:

"How do I activate my new iPhone with cellular service"

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Transferred the SIM card, Yet?:

Remove the SIM card form the old iPhone, and then insert it into the new iPhone. You should be set from there.