[ajayyy]

There is a whitelisted set of apps in the OS. Samsung's messenger also is allowed on Samsung devices. They talked about opening the API many years ago and then suddenly stopped talking about it.

For most people, Google Messenger app now uses Google's RCS gateway anyway, so the carrier does not need to support RCS.

I do not know of anyone trying to get carrier-based RCS working with a non Google app using a custom rom with a different whitelisted set of apps, but it sounds to like maybe that would be possible

[jwells89]

Additionally, only Google’s is capable of encrypted RCS as far as I’m aware (as the standard does not include that), so using other clients means giving up encryption.