Choice to link or not to link iphone’s

• You can transfer nothing from old to new, and use a new Apple ID.
• You can transfer contents from old to new.
• You can transfer contacts and share Apple ID on both devices.
• If you switch Apple IDs, purchases and subscriptions will not transfer, and iCloud contents will require manual move.
• If you migrate contents and share the same Apple ID, things will work as they have, on the new iPhone.
• None of this is particularly related to the carrier, nor to carrier security.
• You will want unique and not re-used passwords, and a not-reused carrier passcode.
• there aren’t particularly good defenses against carrier hacks, other than additional steps including two-factor tokens where available and enabled, using a recovery key (which has its own hazards), and being wary about phishing and related shenanigans.

There is nothing called an Automatic link between one iPhone to another.

If both are your devices then using two different Apple IDs may not give you the benefits of having two iPhones.

If two different people will be using them then it is always recommended that both of them use different Apple IDs.