Freeze one failed call and the ordered topology
Record local and UTC time, calling and called numbers, direction, call or ticket identifier, ordered service, customer source, assigned Colt SBCs, trunk group, peer order, POPs, access paths, PBX nodes, transport, call limits and configured distribution mode. Preserve the first failed call, later attempts and the first recovery call before changing routing or restarting equipment.
Classify the earliest divergence: no provider attempt, wrong trunk selected, request reaches the edge but not the PBX, failure detection is slow, a trunk remains blacklisted, recovery does not occur, diversion triggers unexpectedly, TLS fails, SDP disagrees or media flows one way.
Separate path diversity from routing order and capacity
Draw each trunk from Colt SBC and POP through access, firewall or SBC to the actual PBX node. Mark shared circuits, addresses, devices and power. Compare outbound customer routing with the ordered inbound active/standby, round-robin or proportional mode; they are controlled on different sides.
Retain the configured call limit and concurrent calls at the incident time. The technical guide says Colt can advance an inbound call when a trunk reaches its limit, so a capacity transition can resemble a reachability failure without proving that either trunk was unreachable.
Prove the failure trigger and first-call timing
For UDP, compare the original INVITE, the guide's published 1.5-second retry and single retry with the actual timestamps. For TCP, preserve connection setup, failure and the documented five-second timeout. Record the provider response or absence, customer-edge receipt and the next-trunk attempt. Do not infer a timer from ringing delay alone.
The first failed attempt may incur detection delay before a trunk is blacklisted; later attempts can advance differently. Keep those observations separate, and verify the assigned profile before treating guide values as the account's effective timers.
Prove blacklist exit and recovery mode
Identify whether the order uses the documented 180-second recovery timer or optional OPTIONS probing every three seconds. For timer recovery, observe beyond the actual interval and retain the next routed attempt. For OPTIONS recovery, preserve request, response source, 200 result and subsequent call selection.
An OPTIONS response is narrow reachability evidence. It does not prove number routing, identity acceptance, codec agreement or two-way audio. If the peer remains unused, compare provider blacklist state, transport reuse, PBX route and capacity before forcing a broader change.
Separate trunk failover from diversion and disaster recovery
If the call diverts, preserve every attempted trunk and prove that all were unavailable for the documented non-reachability trigger before attributing the result to inbound call rerouting or partial-number replacement. Compare unconditional, busy and no-answer rules independently. A reachable trunk with a missing PBX route or silent media should not be described as the same failure.
Confirm the destination and identity presented on the diverted leg, expected cost and emergency consequence. Portal activation, revocation and role evidence show configured intent, not the actual call path.
Trace TLS, SDES SRTP and RTP in both directions
For a protected call, retain TLS peer name, chain, mutual-authentication result, cipher and SIP exchange, then compare the SDP crypto attributes and selected media endpoints. The technical guide says MKI is unsupported; if an endpoint offers it, capture the negotiation rather than assuming encryption succeeded because signaling used TLS.
Map packets and decoded audio separately in each direction. Correct only the demonstrated certificate, transport, SDP, MKI, codec, address, NAT, firewall or PBX-media defect, then repeat one authorized call. Do not present a successful retest as universal compatibility, service availability or a provider quality measurement.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- Colt SIP Trunking data sheet ↗
- Colt SIP Trunking product page ↗
- Colt SIP Trunking External Technical Guide, version 2.1 ↗
- Colt: how to access your voice applications ↗
- Colt Companion service assurance ↗
- Colt SIP Trunking support catalogue ↗
- Colt Voice Product News ↗
- Colt CLI Presentation archive ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.