Freeze one inbound attempt and the effective objects
Record the dialed Plivo number, timestamp and timezone, account and data region, inbound trunk ID, enabled state, linked-number evidence, primary and fallback URI, FQDN or IP, port and transport, URI authentication setting, Plivo call UUID, SIP Call-ID, every provider attempt, PBX match, answer, media and caller outcome. Redact credentials and customer identifiers.
Classify the first disputed boundary: number assignment, trunk state, URI value, DNS, reachability, authentication, primary SIP result, fallback attempt, PBX route, answer or media. Do not change all of them together.
Prove number assignment and URI state before failover
Confirm that the called number is linked to the intended inbound trunk and that its primary and fallback URI values match the current design. Resolve each FQDN from the relevant network and preserve the answers and observation time. Verify firewall and NAT policy against Plivo current signaling and media ranges.
Do not wait for REGISTER: Zentrunk does not support it. OPTIONS, DNS and a reachable port are useful but narrower observations; none proves that the called number selected this trunk or that an INVITE matched the PBX.
Distinguish no response from a final rejection
Plivo documents fallback when the primary URI is unresponsive. Trace whether the primary produced no response, a transport error or a final SIP response. If a final response exists, preserve its code, reason, elapsed time and the next provider action instead of assuming that every rejection should advance.
The hangup reference includes customer timeouts, URI lookup failures, routing failures and customer-generated 4xx, 5xx or 6xx outcomes, but it does not turn all of them into one fallback promise. Escalate an observed behavior mismatch with the call UUID and evidence.
Join Plivo debug logs to the PBX record
Export the call details, call stats, hangup source and cause, final SIP response, negotiated codec and SIP PCAP for the same call UUID. Match the SIP Call-ID and timestamps to PBX logging. If the provider shows delivery but the PBX does not, inspect destination address, transport, firewall and packet arrival before changing the dialplan.
If the INVITE arrives, preserve endpoint identification, Request-URI, called number, authentication result, first matching route and PBX response. Provider logs and PBX logs can disagree in scope without either being the complete path.
Treat answer and media as later boundaries
A 2xx response proves an accepted SIP transaction, not that the intended endpoint rang, the caller heard the right treatment or both audio directions worked. Record PBX channel or dialog evidence, SDP offer and answer, selected codec, RTP endpoints, packet counters and decoded audio in each direction.
Keep TLS and SRTP configuration distinct from observed negotiation. The provider-side signaling PCAP does not establish the customer-side media path, and a codec label does not establish DTMF or speech quality.
Correct one owner and retest recovery deliberately
Correct only the first demonstrated owner: number link, trunk state, URI, DNS, authentication, firewall, PBX endpoint or route. Repeat authorized calls for normal primary delivery, an unresponsive primary, fallback delivery, primary recovery and return to primary; test a reachable primary rejection separately. Preserve rollback and remove temporary captures and elevated logs.
One successful sequence does not establish universal failover, active-call survival, capacity, price, availability or compatibility. No Plivo account, trunk, URI, PBX, packet, media stream or live call was inspected or tested for this diagnostic.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- Plivo Zentrunk quickstart ↗
- Plivo Zentrunk technical specifications ↗
- Plivo SIP Trunking account limits ↗
- Plivo Zentrunk hangup codes ↗
- Plivo Zentrunk debug logs ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.