Reference / Voice
Session borders in VoIP: define the edge before routing calls
At an interface between independently administered voice networks, document which requests, identities, session descriptions and routes are accepted.
Keep signalling and media visible
SIP controls session creation, modification and termination; SDP describes the multimedia session. Trace signalling and media separately. A completed signalling exchange does not establish that the required media path is working.
Define admitted peers, authentication, number formats, codec policy, media address handling and failure behaviour. Use explicit routing rules, and record why any alternate route is permitted.
Operate with evidence
Capture call attempts with time, direction, selected route, result and a privacy-appropriate correlation identifier. Keep changes reviewable and test limited inbound and outbound cases after each change.
Topology hiding and address rewriting do not replace access control or logging. Protect management, segment the service, define rollback and agree who investigates a failed priority call.
Test failure paths
Test rejected credentials, invalid number formats, unavailable peers and a media path that cannot be established. A boundary policy is understandable only when its rejects are observable.
Use synthetic test calls only where authorised, and label them so call records and operations staff can distinguish them from real traffic.