How do I transfer phone number from iPhone 7 Plus with iOS 15 to iPhone 12?

The iPhone 12 had a physical Nano-SIM slot. All the OP has to do is remove the SIM from the old phone and put it into the 12.

They will probably need a new SIM to get 5G service, but the existing SIM should work fine assuming the 12 is unlocked or locked to the same carrier as the 7.

Contact your carrier for help transferring your phone number. You do not need iOS 16 or higher to transfer your number.

top185 wrote:

Yes, you can search “transfer“ on settings and follow guidelines on this.

I believe you are referring to phones that have a eSIM. The iPhone 7 Plus still has a physical SIM. Kurtosis12's response was correct. The OP needs to contact their carrier.

Yes, I was referring to contacting the carrier for a new SIM. I was just referring to that poster that they didn't need to search for transfer.

Yes, you can search “transfer“ on settings and follow guidelines on this.