The latest carrier update isn't related to the 10.3.2 betas, more likely related to roaming updates like the last update. Two of my iP6S handsets anchored in the US and on 10.3.1 received the update today (5/7), but the one that I travel to Canada with received an update a couple of weeks ago.
28.4 has been out for a couple of weeks, but only was pushed to my handset that roams. I was wondering then and found that link. I'm surmising the rest of ATTWS's handsets were "eligible" for the update 5/7.