B2BUA (Back-to-Back User Agent)
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A SIP entity that acts as both a UAC and UAS simultaneously, terminating one SIP leg and originating another independent dialog — used by session border controllers, PBXs, and voicemail systems. Unlike a proxy, a B2BUA maintains full call state and can modify SIP messages between the two legs. This allows codec transcoding, DTMF conversion, and media anchoring. The trade-off is that B2BUAs become a failure point for all active calls.