An MVNO, mobile virtual network operator, is a carrier that sells service under its own brand but runs on another operator's network, such as Google Fi, Mint Mobile or Cricket.
How an MVNO fits into messaging
An MVNO owns customers, plans and billing, but not spectrum or towers. Its subscribers' traffic travels the host MNO's network, so for SMS and MMS an MVNO number behaves like a number on that host. RCS is more nuanced: it depends on the host network's RCS platform, the phone's messaging app and, in some cases, whether the MVNO has enabled the feature on its plans. In practice, most US MVNO subscribers with a current Android phone or an iPhone on iOS 18.1 or later receive RCS, because the host operators run Jibe and Google Messages registers directly with it.
What a business sender should expect
Nothing changes on the sending side. The capability check before each message tells the platform whether the handset can receive RCS, and if not the message goes as SMS from the registered fallback number. A sender never has to know which MVNO a customer uses, and should not try to segment by it; the support tracker records where reachability is known to lag.