Alright I tested the link provided by @jiminhno with regards to manually enabling RCS - for reference, I'm with Telstra now and have most of my contacts on Telstra too so we all have RCS.
When I sent a message using Google's Messages app (telstra doesn't support RCS on this app so workaround was used), I didn't have the option to send via RCS even after the app says "Chat" is ready to use - it just sent as SMS. When I tried to send using Samsung's messaging app (RCS is supported on this one), it sent properly as RCS.
I still have my Optus phone and number (post-paid), I'll test out this workaround and see if it works both from my S10 5G (RCS enabled) using the default Samsung messages app and Google messages app to my S9+ from Optus to see if it works.
I'll come back to this post once I have the results whether or not both parties have to be using Google messages app with this workaround enabled or if a person that already has RCS (Telstra) can just use the default messages app and still work fine.
Update 1: Just a quick update, the RCS messages I sent using the Samsung messages app doesn't show up on Google messages app.
Thanks for the additional info - now I'll just be testing if it works properly if both parties are using Google messages with the hack enabled. If that's the case then, this would mean sending the instructions to your non-technical friends/family..
Optus is an expensive telco. They're barely cheaper than Telstra and they've upped their prices again and again. So how come we are paying virtually the same price and not getting all the features Telstra offers? Not only do they have RCS chat which is a game changer for everyone on android, offering an iMessage experience for people with non-apple phones and allowing people to communicate without having to download another app and sign up to another service -- but Telstra also offers video calling baked into the Samsung calling app, it's just as good as FaceTime (and although it needs to be rolled out to more devices) the fact that they can support both RCS and HD Video chatting on Android months and months before Optus, and Optus still hasn't peeped a word about it -- is unacceptable. We are paying top dollar for a subpar experience, we want the latest features and technology on Android and Optus shouldn't be the gate keeper. Carriers in the USA are notoriously more money hungry and anti-consumer than carriers in Australia, and even they have banded together and created an association to develop a unified RCS chat system across all of Android. They saw the monetary potential of businesses being able to RCS message consumers and jumped on the idea instantly. Why hasn't Optus? Give us RCS chat on all Android devices that support it. Whether that be through Google Messages or Samsung Messages. Give us baked in Video chat in the dialler app. Stop using the excuse that you're monitoring the rollout overseas to see the feasibility of it, that's a cop out.
Both participants need RCS enabled, not necessarily the hack enabled. Telstras RCS is weird, and is not compatible with any other carrier/RCS.
If the hack is enabled, you can message anyone with RCS enabled even internationally. The hack method uses googles servers so it does not incur international sms/mms charges and will not show up on your bill.