Apple:

iPhone Handoff lets you use one phone number between two devices, so you can switch between them whenever you want. It makes it easy to move back and forth between devices optimized for the moment — like weekday business or weekend adventures.

[…]

After iPhone Handoff is set up, one iPhone becomes the main iPhone and the other iPhone becomes the secondary iPhone that shares the number. Your main iPhone should be the one you use most often. Some carrier services, like Call Forwarding and Caller ID, and carrier account management can only be managed on your main iPhone. Contact your carrier for more details.

To move between your main iPhone and secondary iPhone, simply unlock the device that you want to start using — your phone number will automatically become active on that device.

[…]

iPhone Handoff works with Dual SIM as long as both eSIMs are from carriers that support iPhone Handoff. Once set up, both eSIMs switch together simultaneously when you unlock your iPhone.

They say you need to have Messages in iCloud enabled.

Joe Rossignol:

In addition to needing two iPhones running iOS 27, “iPhone Handoff” will seemingly require carrier support. The forum post mentions that T-Mobile in the U.S. and Deutsche Telekom in Germany might be among the first carriers to offer the feature.

T-Mobile:

T‑Mobile is introducing the feature at $5/month.

John Gruber:

This seems like a ridiculous thing to charge for. It’s not adding a separate device to your plan, like adding a cellular iPad or even an Apple Watch. It’s just switching your active eSIM between two iPhones. You can do this for free, now, on any carrier via Settings → Cellular — but it takes a bunch of taps, confirmation on both iPhones, and a few minutes for the carrier to register the eSIM transfer. iPhone Handoff just eliminates all the annoying friction and waiting.

Chance Miller:

As reported by CNET, AT&T seemingly has no plans to support iPhone Handoff.

Previously:

• iPhone Duo

AT&T Carrier eSIM iOS iOS 27 iPhone Handoff Messages in iCloud T-Mobile