UNLOCKED S23 Ultra on AT&T

The phone hasn’t been released yet. I doubt anyone knows yet. My guess would be it will.

I'll echo that.  It should.  Previous releases did out of the box.  

It's risky. While the features you list probably will work, I've seen numerous times on unlocked samsing phones on AT&T networks where the advanced messaging feature is not available. While it may not be necessary if Wi-Fi calling is available and working, sending pictures via regular text messaging (cellular only connection) is next to impossible, if they go through at all it can take up to 5 minutes to send.  My speculation on this is that it's intentional to ensure that you get the phone branded by AT&T, and those phones do have the advanced messaging feature enabled.

What is advanced messaging? Is that the same as RCS? 

Advanced messaging is AT&T's term for Rich communication Services which uses data rather than SMS text.

  There's no reason to be concerned because RCS will work fine on unlocked Samsung's.  I'm on my second unlocked (s21+ to s22)  zero problems.  Everything worked out of the box.

Thank you @formerlyknownas! Very glad to hear this.

I've always purchased carrier-branded phones previously. Given that AT&T, like other carriers, prorates the trade-in credit over 36 months, their deal isn't very appealing if you plan on upgrading your phone before the 36mos are up. So I decided to order the Unlocked S23 Ultra direct from Samsung even with the lower trade-in credit for my S21 Ultra. My calculations show that after 24 months (my typical phone ownership period), the Unlocked variant is about $290 more than what I would have spent at AT&T if after 24 months I decided to pay off the phone and upgrade. Having the flexibility of being unlocked felt worth it to me.

If you look at the AT&T compatibility sheet they publish, it does not have advanced messaging listed with the s22, it has it listed for other models but not that one. Presumably I would assume it probably doesn't work for the s23 either. The feature is necessary if you intend on sending larger files through texting and you are not on Wi-Fi. A simple picture you take on your phone will be large enough it'll delay sending it, to the point it's impractical to use. Otherwise I woild get an unlocked. The other part that (Edited per community guidelines) is AT&T doesn't offer 1TB on their phone, only the unlocked version offers that.

Thanks @pwl789. Advanced Messaging is RCS (Rich Communications Services) and that is essential for me. I've ordered the Unlocked version and it will get returned if RCS is not supported, so time will tell.

If history is anything - everything will work on an unlocked S23 (which I highly recommend over the ATT version) with the exception of ATT Video Call, which will be replaced by Google Meet.  Hotspot, and Wifi calling will work fine.  Software updates will lag - as unlocked phones usually get them last.  Other than that - you won't know the difference.  SMS will use Google's Jibe RCS server, and not ATT's silly RCS implementation.  You won't have any of the ATT bloatware installed, but if you want them - you'll be able to find them on the Play Store.

Of course, this is speculation since S23 is not out - but this is how it is on the S22, and Z Fold 4.

If RCS inter carrier operability is important to you, avoid the ATT version of the S23 because they nerfed it in the S22 to not work with other carriers and not even with ATT customers who have non-S22 phones.  They never cared to "fix" it and there is a huge thread here regarding RCS not working with the S22.  There's probably going to be a new thread for RCS not working with the S23 if they haven't "fixed" it.

Thank you for the informative post @jsy1! I've no interest in AT&T Video Call so that is not a loss, glad to hear the other features should work fine. 

For software updates on Unlocked phones, I assume the responsibility falls to Samsung to push out monthly security updates and new Android releases. Hopefully, they do not have a terrible lag behind the carriers.

I currently have an AT&T branded S21 Ultra and use Google Messages. It uses the Jibe RCS servers and I can use RCS inter-carrier without issue. Had no idea this became an issue with the S22, thanks for bringing it up.

Looking forward to my first unlocked phone experience on AT&T.