RFC 7092 taxonomizes B2BUAs. An SBC is commonly a UAS on one side and a UAC on the other, not a pure proxy.
A back-to-back user agent receives a request and processes it as a UAS. To continue the call it acts as a UAC and generates a new request.
RFC 3261 defines that concatenation. RFC 7092 taxonomizes the B2BUAs that products actually build, including SBCs that also touch media.
A pure proxy forwards the same request and adds its own Via. It does not create a second dialog with a new Call-ID just to hide the first hop.
An SBC is commonly a B2BUA. The outbound INVITE in this synthetic uses Call-ID b2bua-legb-01@example.com while the caller still uses Call-ID a84b4c76e66710@example.com.
Room: An SBC is commonly a B2BUA
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