alt Hacker NewsFrom [0], it seems that RCS can use the cellular signal instead of LTE or 5G:
> In cases where RCS is able to operate over cellular networks without data, it supports messaging as well as file transfer, enriched calling, and more.
[0]: https://en.wikipedia.org/wiki/Rich_Communication_Services
In LTE and 5G there exists no cellular signal without data. That was a 2G/3G thing. Since 4G everything is data. The separate sms and voice services are gone. And 3G is rapidly being deprecated.