It's been there since the start in iOS 8.1 ->
https://www.redmondpie.com/how-to-use-sms-forwarding-in-ios-8.1-on-iphone-ipad-and-yosemite-on-mac/
A more recent example of it in action ->
https://www.seattletimes.com/business/technology/how-to-auto-forward-text-messages-from-an-iphone/
Towards the bottom it says "Oh and this also works great if you have multiple iPhones at hand, each with different carrier SIMs, in which case you also get the perk of receiving SMS / MMS from the other device to all your other devices, including iPads. In short, you don’t have to carry two iPhones in your pocket to reply to an SMS / MMS."
It wasn't great though, I remember going on a trip and my iCloud going haywire and struggling to re-set it all up with the codes for about an hour in the car.
i believe it was two-factor verification that "fixed" the setup process and seemed to make it just work better. I didn't start using it right away iirc, but it was definitely fine by iOS 10.
technically that would be because you can't disable the number on the sim card in the iphone on that device, thus you can't add it - it is already there. i'm not sure how best to explain that one.
I guess this is the difference that I'm not seeing, since I do have a sim card in there. That issue could easily be fixed though with a PAYG sim card.
Sorry to bring this conversation up again out of the blue, but a year later I finally got a chance to test a second iPhone’s Continuity functions, and I remembered this conversation and thought I should update my findings.
In my tests, my main iPhone has an active line and is running iOS 14, and the second iPhone is not activated (WiFi only) and is running iOS 10.
I found that iMessage and FaceTime were fully functional on both and synced perfectly. Although for the second iPhone in FaceTime and Messages settings, it constantly said “waiting for activation”. Not sure if this affected battery life.
Calls and sms messages were successfully forwarded from the primary iPhone to the second iPhone. But I was not able to make calls or send sms messages from the second iPhone. Instead of going through the primary phone via continuity, the second phone seemed to want to make all calls and sms messages using its own cellular radio, which obviously failed as it doesn’t have an active line. Calls resulted in Verizon messages and sms messages failed to send.
Other continuity features were largely unsuccessful.
Handoff partially worked, but not to a useful degree. While my primary phone could open an app that was active on my secondary phone, it could not open the specific document or page. Ie. Notes.app would open but not the specific note I was working on, and Safari would open but no webpage would load. And while apps did partially handoff from the second phone to the primary phone, it did not handoff to either of my two iPads (also on ipadOS 14).
iCloud Safari tabs did not work.
Universal Clipboard did not work.
So those were the results. I couldn’t test all the continuity functions like Instant Hotspot, but I felt these were the most pertinent functions. ideally for the test, I would have had all the devices on the same iOS version just in case, but I have to keep the second phone on iOS 10 for the time being. But I could not find anything saying continuity features would not work across the two iOS versions, so the different versions were not likely the problem.
To conclude, unless apple has fixed this since iOS 10, a second iPhone has very limited continuity functionality. It seems using a second iPhone as an iPod touch is still too niche of a use case for Apple to make the effort to bring full continuity functionality to it. For me personally, I could probably deal with not having most of the continuity functions, but the dealbreaker is the inability to make calls or send sms messages.
). I've used my IpT primarly for gaming. At this point, for $300, I'd much rather just get a Nintendo Switch!