What the feature actually does

Apple describes it as allowing users with supported carriers to easily switch back and forth between two iPhone models with the same phone number. That is a narrower claim than it sounds. It is not two lines and it is not Dual SIM. It is one number that can live on two handsets, with the active one switchable.

The obvious use is a two-phone life: a large phone for the desk and a small one for a run, or a new iPhone alongside the old one during a transition week. Until now that meant either moving an eSIM back and forth through Settings each time, or paying for a second line.

Why a carrier has to agree to it

A phone number is a carrier record, not a device setting. Handing one number to two devices means the network has to know which handset to ring, and has to accept being told to change its mind. That is work on the carrier's side, which is why the feature arrives per-carrier rather than all at once — the same pattern Wi-Fi Calling, VoLTE and eSIM Quick Transfer all followed.

If you're not on T-Mobile

There are two workable substitutes, and neither is as clean.

• Move the eSIM manually. Every current iPhone can transfer an eSIM to another iPhone from Settings. It takes a minute or two and only one phone has the line at a time.
• Add a cheap second line. A talk-and-text or small-data plan on the old phone gives you two numbers permanently, from about $8 to $10 a month. That is a different thing from Handoff — two numbers, not one — but it needs no carrier permission at all.

Should this change your carrier?

No. One convenience feature is a thin reason to move networks, and carrier support for this kind of thing tends to spread within a year. If you are already weighing T-Mobile for coverage or price, treat Handoff as a small point in its favour rather than the reason.