How to transfer the phone number from an old iPhone to a new one?

Unless the new phone is an iPhone 12, take the SIM out of the old phone and put it in the new phone. If it’s an iPhone 12 contact your carrier and ask them to switch the phone number to the new phone. You may have to get a new SIM from the carrier.

Helpful link ➡️ Remove or switch the SIM card in your iPhone or iPad - Apple Support

For an iPhone 12 ➡️ Using Dual SIM with an eSIM - Apple Support

Insert the SIM from the old device to the new one, that's it.