Number lookup is a query that returns what is known about a phone number before a message is sent: its carrier, line type, whether it is reachable and whether it can receive RCS.
Two kinds of lookup
The carrier lookup answers which network serves a number and whether it is mobile, landline or VoIP; it is the check that stops a list from wasting money on numbers that can never receive a text. The RCS capability lookup asks Google's platform whether that specific handset is registered for RCS right now. The RBM API exposes it per number and in batches, and every serious platform runs it automatically as the capability check that decides between RCS and SMS for each message.
Why it is not a one-time job
RCS capability changes. A customer switches phones, turns chat features off, or moves to an iPhone that gained RCS with iOS 18.1. A capability result cached for weeks will send SMS to people who could have had RCS, or worse, attempt RCS to a handset that has dropped it. Checking at send time keeps the fallback decision honest and the delivery rate accurate.
On SimplyRCS
The capability check runs on every send and costs nothing extra; the analytics show how much of each campaign went RCS and how much fell back.