Each header answers a different question
Via records the request's signaling path and supports transaction matching and response routing. Contact gives a target URI for subsequent requests in contexts such as dialog establishment and registration. Record-Route lets proxies place themselves in the path of later dialog requests; those values contribute to the route set used in Route headers. These roles overlap in a packet capture but are not interchangeable. A public Via value cannot repair an unreachable private Contact automatically. Neither header is the SDP media destination.
Diagnose the point where routing changes
A common failure is a successful initial INVITE followed by a missing ACK or BYE. Compare the initial destination with the later request's Request-URI and Route set. Check whether the destination came from Contact, whether the expected proxy stayed in Record-Route and whether a NAT mapping or connection was still usable. Use the same dialog tags when comparing messages. For a B2BUA, repeat the analysis separately on each leg: headers may be regenerated rather than forwarded. An apparent mismatch across the PBX boundary can be intentional.
Avoid indiscriminate rewriting
Document which component owns public signaling addresses, connection reuse and media relay. If both an edge device and a SIP ALG rewrite messages, the second rewrite can undo the first component's correct work. Capture before and after the boundary to locate the alteration. A useful trace note includes the request destination, top Via, Contact and ordered Route values, with credentials and personal identifiers removed. Keep SDP in a separate media-path note. This prevents a signaling fix from being mistaken for an RTP fix.
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.