Freeze one number, call and expected continuity mode
Record one authorized non-emergency inbound attempt with timestamp and timezone, called number, caller result, expected PBX endpoint, actual PBX endpoint, SIP Call-ID where available and whether the primary was intentionally healthy or unavailable. Export the current trunk set, number route, endpoint list and selected high-availability mode before changing anything.
“The backup did not ring” is not one diagnosis. Under Primary/Backup a healthy primary can make that correct; under Round Robin one call can select the other endpoint; under Forking both can receive branches; PSTN backup does not target a PBX endpoint.
Prove trunk-set membership, number routing and endpoint order
Match the called number by the longest configured prefix to its number-routing object and trunk set. Confirm that the expected PBX endpoint is active in that same trunk set, uses the intended template and has the required priority or enabled state for the selected mode.
Do not infer membership from a registration, DNS record or identical extension plan. NFON documents endpoints and number routing as separate objects, and Primary/Backup order is controlled by endpoint priority.
Classify endpoint readiness without manufacturing an outage
For credential mode, correlate resolver output, transport reachability, REGISTER, challenge and final response, Contact and expiry. For static mode, correlate the approved public address, observed source and portal setting. Then obtain provider-visible endpoint state or a ticket timestamp for the same incident window.
NFON describes Primary/Backup selection when the primary endpoint is not ready, including unregistered or faulty states, but the public sources do not publish one universal detection timer. Do not drop arbitrary packets, disable all security controls or place emergency calls to force the condition.
Follow the selected branch into the PBX
For the chosen call, preserve which endpoint NFON selected, network arrival, INVITE, provisional and final responses, Request-URI, identity, first PBX route and final application outcome. A provider selection does not prove edge arrival, and an INVITE at the edge does not prove that the backup PBX owns the same extensions or route.
If Forking is selected, correlate both branches and the winning answer plus cancellation of the other branch. If Round Robin is selected, observe a bounded series rather than declaring failure from one call.
Separate PSTN backup and active-call behavior
NFON documents a separate backup service that can forward failed inbound routing to a PSTN number. Verify its number-specific or prefix-specific configuration independently from multi-PBX high availability. A mobile or alternate PSTN destination ringing is not proof that the secondary PBX was selected.
The public documents describe treatment of new inbound attempts; they do not establish migration of an established dialog, media stream or in-progress call. Record active-call survival, if required, as a separate provider-approved test objective.
Retest one reversible transition and restoration
In an approved window, use the provider-agreed method to make the primary endpoint unavailable, observe its NFON-visible state, place bounded non-emergency calls, and verify selection, PBX route, identity, audio, digits and clean release. Restore the primary, observe readiness, and confirm the documented return behavior for the selected mode.
Stop and roll back if detection or selection differs from the plan. Preserve the ticket, timestamps and results without inventing a failover timer, claiming active-call preservation or converting a catalogue feature into laboratory validation.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- NFON SIP Trunk Flexx product page ↗
- NFON SIP Trunk Flexx technical manual v3.1 (DE) ↗
- NFON SIP Trunk Flexx SLA v1 (EN) ↗
- NFON service status ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.