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If an SMS sent to your new phone with your old number gets to you, then pretty clearly SMS was not hijacked.

On the contrary, Messages is routing messages to Messages IDs, and it's kinda on you whether you use an ID string that looks suspiciously similar to your number.

This is incredibly easy to turn on and off:

https://support.apple.com/en-us/HT201349

To be clear, you can have an iphone with an SMS phone number and Messages, and not ever mingle the two. Further, you can have a SMS phone get your SMS messages, while Apple users with your number reach, say, your iPad. That's a valid use case.

In the situation you lay out above, where you forgot you didn't want to use that as your Messages ID any more, you can "de-register" if you forgot to remove it:

https://support.apple.com/en-us/HT203042

https://selfsolve.apple.com/deregister-imessage/

>This is incredibly easy to turn on and off

If only if it worked as advertised. I can turn it off as to messages directed only to me. But if a group text was on iMessage and I'm the only one who bounces off to Android, new messages to that thread will exclude me, EVEN IF I'VE DEREGISTERED MY NUMBER. I've tried this repeatedly. The only way to begin receiving messages from that group is if EVERYONE on the thread deletes the old thread and begins a new one in green bubbles. It ENRAGES me, and it's been the biggest barrier to me transitioning to a Galaxy phone. When I'm the only one on Android, I just stop receiving the messages to the legacy group threads. And there's no way I make 30 iPhone users all delete those threads. Apple could ask its own servers "have any of these iMessage accounts deregistered their number? If yes, force thread back to SMS/MMS." Apple does not do that, and thereby severely handicaps any effort to move away. I hate it and would happily support a class action to change that practice.

That’s not how it works. When an iMessage cannot be delivered for any reason (phone changes to Android, no internet, etc.) the message will be sent as SMS within minutes. It works per message and this behavior can be toggled in settings. I personally toggle it off so that I don’t get accidentally charged for SMS. When iMessage fails it tells me under each message’s bubble so I can send via SMS by tapping on it if I like.

I’m not saying this flow is good, but you can deregister and disable iMessage, even if you don’t leave iOS, but especially if you do: https://selfsolve.apple.com/deregister-imessage/

It’s not intuitive of course, but you can totally turn it off and deregister, even if you don’t have your iPhone anymore.

This only works for 1:1 conversations. If you were in any iMessage groups and then deregister, iOS will silently drop you from future messages, and it isn't possible for the same group composition to be re-established over MMS.

SMS/MMS is insecure, so ability to replace an authenticated iMessage user with an SMS recipient in existing groups would endanger the whole groups.

It could at least ask the user the question. "One of the users on this thread has deregistered their number. Would you like to send by SMS so that they are included, or are you comfortable excluding them from this conversation? The SMS message would not be encrypted."

That makes sense since SMS doesn't support iMessage's always-on encryption but normally you can recreate the group using SMS once the former iMessage user's deregistration has gone through.

No, you can't do that. Once you've been a member of an iMessage group, iOS will not allow participants who have iMessage enabled to switch to MMS with that same group of users, even if everyone deletes and re-creates the thread.

That problem is just with an Apple user to an ex-Apple user. In other words it's a problem that Apple exclusively created and they benefit from not fixing it. It just means a current Apple user goes to text you and Apple thinks you are still using imessage so instead of sending an SMS message it sends it via their imessage service. Apple likely makes this process a pain to make it harder for people to switch OS'es and makes you more locked in. Not only are you going to lose your imessage groups, but you'll also have difficulty for awhile with getting messages properly.

> All messages from other iPhone users to my phone number are routed incorrectly and I don't receive them until Apple updates its gateway to use SMS for my phone number.

iMessage falls back to SMS within a few minutes if the message can’t be delivered. This often happens if the recipient doesn’t have a data connection.

I've also noticed that once you have an iphone if another iphone user adds you as a contact it stores your number as "iPhone" in the contact card type.

When I've switched back and forth from Android on a few occasions I've had to have users update that contact card for SMS to work properly. I don't think its necessarily an iMessage issue more iOS, but if anyone is still having issues with the switch after de-registering, check that.

hijacking

How exactly is Apple supposed to know that you have switched phones? Any way I can think of requires privacy intrusions that make me shudder.

However, they do have UI on all of their iMessage programs, and on the website that you can get to from any web browser (on any brand of device) for your AppleID/iCloud account that can remove your phone number from your AppleID at any time. Then, after some minor propagation time, messages from Apple users to your phone number will no longer be routed to your iMessages account, and will instead be sent as SMSs.

So you have an option to tell Apple at any moment, even after you have switched, that you no longer want that number attached to iMessages. How exactly is that hijacking?