An MNO, mobile network operator, is a carrier that owns its radio spectrum and network infrastructure. In the US the MNOs are AT&T, Verizon, T-Mobile and UScellular.
The MNO's role in RCS
RCS is a carrier standard, so every RCS message is ultimately delivered by an MNO's messaging platform or by the hosted platform it has adopted. The three national US operators and UScellular all run their RCS on Google's Jibe platform, which is why a single Google-verified agent reaches subscribers on all of them; the carrier support page and the support tracker hold the current picture. MNOs also set the commercial terms: per-message rates, 10DLC campaign fees and the pass-through surcharges that appear on an SMS bill.
MNO versus MVNO
A mobile virtual network operator resells an MNO's network under its own brand. Its subscribers ride the host network, so RCS reachability follows the host, but plan and device restrictions can still differ.
On SimplyRCS
SimplyRCS routes RCS and its SMS and MMS fallback over direct connections to the four US operators, with each operator's fees listed at cost on the pricing page.