Comment by jjtech

> Assuming your carrier bothers to run RCS, the protocol works just like MMS and SMS do. If your operator doesn't peer with other operators then you'll have the same issues getting any kind of multimedia delivered from phone to phone.

Except, SMS/MMS can be implemented by any standard IMS stack and it will function on any carrier (in theory, ignoring implementation bugs/incompatibility);

RCS has an explicit provision in the standard for "client authenticity" checks which in practice means App Attest/Play Integrity signatures.

(see also: my comment above)