My mum and I both have identical phones - A71. However, I have RCS (Rich Communication settings) on my phone, she doesn't. It simply does not exist on her phone. We can't figure out why.
We both have Samsung Messages set as our default messaging app.
We both have Android version 11.
Her version of Samsung Messages is 12.1.60.17, mine is 12.1.60.13. Strangely both say "The latest version is already installed."
On the "About Messages" screen, mine shows 3 buttons - "Learn more about chat features", "Chat Terms of Service" and "Open source licences". Hers only shows one button, "Open source licences".
I doubt this is significant, but she is on Talk Talk, I'm on Post Office broadband.