iOS seems to have a well-known / generic default Jibe endpoint that profiles use without a specified endpoint. Technically nothing stops Apple from enabling that for every single provider out there, mimicking the default fallback in Google Messages.
People going through carrier profiles found a lot of providers that don‘t specify any endpoint and let iOS use the generic one (they just provide a flag that enables RCS without specifying an endpoint), which is not tied to a single cellular provider. I think Telekom.de is one of them, using the generic RCS EU sandbox jibe endpoint.
Unsure whether these providers have agreements with Google or whether that is an essentially free way of unlocking RCS, since I doubt Google vets any connections / sign ups that way (people used a similar method years ago to sign up for Jibe RCS on their phones, hacking in a generic sandbox endpoint, before Google Messages added the default fallback).