Well, the situation is a bit more complicated, so let me explain the differences between iOS12 and iOS13 as I experience in the context of my configuration, and what I had and have to do, to have multiple phone numbers associated with my Apple ID.
So prior to dual-SIM iPhones the process of having multiple phone numbers attached to an Apple ID was simple: buy multiple iPhones, have the corresponding SIM in each phone, set up iMessage with the AppleID, and that was it: each phone, Mac, iPad, etc. could receive messages from all numbers. Keeping things in sync was an issue, when a device was powered off for some time.
With iOS12 and messages in the cloud, that sync problem went away, and the appearance of dual-SIM phones seemed to reduce number of physical phones required. In my case, the two main numbers are a US and an Austrian number, and carrying two phones around at all times gets old quickly, so I jumped on the XSmax.
It turned out however, that one had to designate one number as the number which is registered with iMessage and FaceTime, and that was a bummer. Fortunately my Austrian carrier allows, for a relatively small fee, to get additional SIM cards for the same phone/data line. Mostly this is used for things like phones built into cars, but with the arrival of eSIMs that’s also what’s used for smart watches, etc
However, while all cards can make and receive calls and use the data allotment, only the main SIM can directly send and receive SMS, which is a requirement for iMessage registration. So I ended up making the eSIM in my iPhone XSmax the secondary SIM, and made the physical SIM the primary SIM (one can switch the role of the SIMs at any time in the carrier’s customer portal, which is key!). Thanks to messages in the cloud, I didn’t care which card would actually receive the SMSs, as they would pretty much instantly sync over the cloud anyway, and actual iMessages are not affected by that in the first place. So then the configuration was: old iPhone with primary SIM at home, registered with iMessage, iPhone XSmax with physical US SIM and Austrian eSIM is registering the US number with iMessage, but picks up the Austrian and other numbers as iMessage always had. So aside from the €5 additional monthly fee for the second SIM, things worked just fine.
Now, enter iOS13: The setting for which number to use for iMessage is gone. But also I noticed, that while iMessage registered the Austrian number just fine on the old iPhone, the XSmax would be incapable of properly registering the number, it wouldn’t just pick it up as registered elsewhere: it did recognize that it had a SIM with the same phone number, and tried to register it directly.
So then I switched in the customer portal, which SIM is the primary SIM, and made the eSIM the primary SIM. Sure enough, the Austrian number now registered fine with the XSmax, but no longer with the old iPhone. The US number is (so far) unaffected. In other words, here are the differences between iOS12 and iOS13:
Can iMessage/FaceTime pick up additional registered phone numbers, that are registered in other phones, for which the phone in question has no SIM?
Both iOS12 and iOS13: yes
Can iMessage/FaceTime register more than one phone number with a single phone?
iOS12: no
iOS13: yes
Can iMessage/FaceTime pick up the iMessage registration of a phone number for which the device in question does not contain the primary SIM?
iOS12: yes
iOS13: no
So while I’m basically looking forward to getting rid of the extra €5/month subscription fee for the second SIM card, and while things on my XSmax finally seem to work now as I originally hoped they would when I bought the device close to a year ago, the forced use of a secondary SIM card offered use scenarios that I somewhat got used to: When I wanted to leave the house light e.g. for going out in the evening, an iPhoneSE was more suitable than the XSmax on which I rely most of the day. So having a second, small phone for such cases was nice to have. Unfortunately now, that the phone with the secondary SIM won’t properly register with iMessage anymore, that use case is essentially made impossible.
Apple needs to recognize if a number is already registered by a primary SIM by another phone, and in that case, pick up the iMessage registrations without trying to re-register the phone’s phone number directly. Not sure if there’s a way for iOS to query the SIM on whether it’s a primary or secondary SIM, though, so this may be impossible.
Hope this explains the differences, and also my original question, because I wasn’t sure if the old iOS12 setting was just hidden somewhere.
