A B2BUA terminates and originates signaling
A back-to-back user agent acts as a user agent server on one side and a user agent client on another. It maintains associated call legs rather than simply forwarding one end-to-end SIP dialog. A PBX commonly uses this pattern to apply dialplan logic, bridge calls and manage features. The legs can have different Call-IDs, tags, codecs and security policies. To investigate a failed call, correlate both legs through the intermediary's records instead of requiring headers to match end to end.
An SBC describes a border role
Session border controllers can enforce signaling policy, hide topology, handle interoperability and anchor media. Their capabilities vary. An SBC is not synonymous with an ordinary firewall, and deploying one does not automatically fix every NAT or security issue. Media anchoring can make a controlled path observable and enforceable, but adds bandwidth and availability responsibilities. If the border terminates encryption, understand who can access plaintext and how the next leg is protected. Hop-by-hop TLS does not become end-to-end media encryption because the box is called an SBC.
Choose explicit responsibilities
List which component authenticates endpoints, owns registration, normalizes numbers, limits destinations, relays media and retains logs. Avoid competing rewrites across a phone, PBX, SBC and router ALG. Test call setup, early media, hold, transfer, DTMF and failover at the actual boundary. Measure concurrent signaling and media load separately. A standby that accepts new registrations may still lack the state needed to preserve established dialogs. Document that limit in the recovery design rather than promising seamless calls from a high-availability label.
Watch the explanation
Fred Posner explains how SIP routing, registration ownership and media services can be split across Kamailio, Asterisk and RTPengine. The discussion of backend probing and reconnect surges helps frame a larger deployment.
Read applicability and editorial notes →Watch this video here
AstriCon 2021 talk using Kamailio 5.5 examples. Treat it as an architecture lesson; our current written cluster uses 6.0 documentation. Review used the available transcript and primary references; audio and screen readability remain unverified.
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Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.