It looks like they're using US Mobile (which resells T-Mobile as "Death Star"). IIRC US Mobile has some big gaps, I wouldn't be surprised if RCS if one of them. With their rebadged Verizon service you don't even get Visual Voicemail
That’s what I thought too. The author appears to have tried every obvious debugging step, except for switching away from US Mobile.
I’ve been using US Mobile myself for a little over a year, and I remember a period of about 2-3 months where most carriers had implemented RCS for iOS on their services, but US Mobile had not, so I couldn’t use RCS for a while. I don’t know what they had to implement to get RCS working on iOS, but it’s possible that their implementation does not work with iOS 26.
