macOS messages to T-Mobile with RCS enables fails (reliably for one user)

For one particular friend (who is on Android with T-Mobile) I have this 100% consistent failure situation:

1) From MacOS, send a message. After a minute or two the message is marked as "Not Delivered".

2) He however has received two copies, one via SMS and one from RCS (apparently)

However, if I send it from my iPhone, it works fine as near as I can tell. As a test, I turned off RCS on my iPhone, and macOS Messages seems to be behaving better. (But of course, this is not good solution).

Also, I do have other friends on Android that I _think_ are on T-Mobile for whom I've not seen this problem.

MacOs: 15.2

iPhone: 16 Pro with IOS 18.2.1