With RCS, you’ve got multiple components to deal with. There’s the clients, but there’s also the servers. RCS is tied to your phone number and IMEI. This means that it’s the telcos who route the messages.

But unlike SMS, where the line level protocol had a back channel designed to transmit these short messages, RCS goes over LTE, which means it needs a network server to send and receive these short messages (more like email).

This means that someone needs to set up and manage the servers for each telco, and route the messages from there to the related numbers.

In the US, the vast majority of ISPs have hired a third party to handle this for them. That third party is Google.

So while the message contents are e2e encrypted, the trunking information and encrypted data can all flow through Google’s servers, even if both recipients are using iMessage, if they’re communicating by RCS instead of iMessage protocol and their telco uses Google for RCS trunking.

If you want RCS, you have to go with one of the corporate apps like Google Messages or Samsung Messages. It’s sad and I hope the situation changes eventually because RCS is much better than SMS and more ubiquitous than signal.

Basically the only option is to use Google messages on Android and the messages app on iOS.

RCS is hypothetically open, but in practice those are the only two respected implementations. Neither Google nor Apple has given sufficient API access to make RCS work outside of their respective apps on their respective platforms.