> They likely won't be able to connect to their mobile network's RCS

And why is that? 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.

> It is. The Google's RCS endpoint requires attestation that is available through Play API only.

Annoying, though hardly unexpected at this point. Still, that only poses a problem for Google's RCS servers, not for RCS itself.