Question regarding an old device that was gifted

Did you not sign out of your Apple Account on the iPhone before you gifted it to your nephew?

If its still listed ion your devices, then its still linked to your account. Which should not have happened.

If you did not, he'd still need to ease it and set it up again, before he can use it as his own.

The child Apple account should not affect it.

click here ➜ What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If there is enough iCloud storage, have him back it up to iCloud first and then erase it.

Archive or make copies of the information you store in iCloud - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

I would hope that restoring from an iCloud backup would not just put your name on the device again.

You don't say what kind of preparation you did for a new owner. While I think that entirely resetting as the earlier cited article describes is good advice in normal situations, in this particular situation you may be able to get away with simply removing the device from your Find My so he can enter his account in that feature. You can remove it either on your former device yourself, or remotely.

Turn off Find My on your devices and items - Apple Support

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you no longer have your iPhone or iPad

If you're using iCloud and Find My [device] on the device, sign in to https://icloud.com/find or the Find My app on another device, select the device, and click Erase. After the device has been erased, click Remove from Account

For him: How to turn on Find My on your iPhone, iPad, or Mac - Apple Support

Note that if you do not remove it from your list of devices there isn't any real harm other than it will never be 100% his. There could be situations where leaving a different person signed in might have advantages if the new owner is a very young child and you want to have ultimate control over the device.

The phone line has nothing to doing with that.

If it was using a SIM card, you should have removed it. If it was using an eSim it should have been deleted when you reset the phone.

If steps were correctly followed, then it should not appear there at all. I think you missed a step somewhere.

If you go into the Find My app, can it still pin-point its location?

Deleting it from your Find My will effectively make it his device (he may then wish to sign into Find My so that is completed).