Google Chat FeaturesRCS status stuck at: Setting Up, Trying to Verify my phone number

I have an unlocked S22U and Pixel 6 as well and it works fine for both.  Google support forums have thousands around the world with your issue.  Especially when playing around with dual sim.  It's not T-Mobile servers causing the problem.  Some people had to leave their sim in a single device, and/or stop using dual sim and wait then magically it fixed itself.

Google didn't invent RCS.  They just bought out Jibe and began marketing it heavily. Google messages isn't modified.  Only carriers did the right thing.  Google services combined they have your whole history and if you think they can't Identify you that is a major problem.  They have your browser, location, view, phone number, who you text, your bank/card account, devices you use, and a slew of other things.  Remember you are the product for Google.

Did you disable RCS on your Pixel 6 before switching?  Also did you turn off RCS server side on Google?  I posted another post before your reply it's under moderation.  Try going to Google support forums for messaging there should be a sticky/pinned where you can disable RCS server side.  Just give it a few.  Clear Cache on both devices, and then try turning on RCS on your S22.