T-Mobile charges $5 a month to run one phone number on two iPhones

The person this is built for owns two iPhones and wants one number to ring on whichever one is in hand. Engadget names a migration between an old phone and a new one, and a cheaper, less valuable phone carried on a trip in place of a $2,000 iPhone Duo ^[14].

None of it does anything until the carrier turns it on ^[4]. Engadget's framing is that you stop running a separate carrier account for the second phone ^[12]. T-Mobile charges $5 a month for Handoff, which is $60 a year ^[6][18]. Whether that saves money depends on what the second account costs. Engadget does not give that figure ^[19].

Setup happens on the phone, under Settings then Cellular, where you choose "Use Number on This iPhone and Another" and assign the main device ^[16]. Apple says that removing Handoff completely means contacting your carrier ^[11]. So the person who supports these phones for other people can switch the feature on from the handset and has to phone the carrier to switch it off. Once it is running, a small indicator appears in the Dynamic Island just under the Wi-Fi icon ^[17].

Calls, messages, and payment cards added on one device all carry across ^[3]. The rest of the split follows the eSIM: the primary phone holds the main one and manages carrier settings, and using each phone's own eSIM requires both to come from carriers that support the feature ^[8].

Corporate mobility policy sits outside all of this. Engadget's explainer never gets into employer policy or corporate accounts ^[19]; the closest it comes is naming professionals who use different devices for work and personal life as a possible audience ^[15].

What settles availability first is the carrier attached to the number, and with the confirmed list at two, most people stop there ^[5]. Then there is whose account absorbs the $5, because the payer is also the party who has to call when the user wants the feature gone ^[6][11]. Engadget calls iPhone Handoff a niche feature ^[13].