Freeze one failure and the exact trunk object
Record UTC and local time, account and trunk labels, portal-issued proxy and username with secrets redacted, PBX product and version, transport, public and private addresses, called and calling numbers, direction, expected route, response, answer and audio result. Preserve the last known good time and recent PBX, router, ISP, firewall, DID or portal change.
Classify the first unexpected boundary: portal object, DNS, UDP request, REGISTER response, blocklist, provider attempt, PBX receipt, DID match, outbound identity, dialog or media. Do not copy values from a different account or dated screenshot.
Prove the UDP registration exchange
Compare the loaded SIP proxy, registration username, re-registration interval and UDP transport with the selected portal record. Capture the first REGISTER and response at an authorized observation point, including source address, Contact, expiry and response code without exposing the password.
A missing registration may come from a wrong credential, wrong proxy, TCP selection, DNS, route, firewall, blocklist or NAT state. A green PBX icon is a useful observation but does not prove inbound DID delivery, outbound authorization or media.
Check blocklists and delivered DIDs before broad firewall changes
MaxoTel documents a 3CX pattern in which traffic for unconfigured DIDs can be classified as unauthorized and lead to SIP-server blocklisting. Correlate the exact source, delivered DID, PBX security event and provider attempt before removing a block. Use current provider address guidance rather than assuming the historical addresses in the article are complete.
Do not disable unrelated security controls or allow an unrestricted network range. Record the exact rule, reason, approver, test and rollback.
Separate SIP ALG, NAT and router-capacity hypotheses
Confirm whether SIP ALG changes signaling, whether multiple devices reuse a source port, and whether the NAT mapping remains stable across the registration interval. Compare the PBX-side and provider-side transaction where authorized.
MaxoTel lists registration loss, missed ringing and one-way or no audio under both SIP ALG and NAT guidance. Those symptoms do not identify the mechanism. A reboot that temporarily clears a NAT table is not a final root cause or permanent fix.
Trace DID, caller ID, signaling and media independently
For inbound failure, prove the provider attempt reached the PBX and inspect the delivered called number before changing the route. For outbound failure, preserve Request-URI, selected trunk, authorized caller identity, Call-ID and response sequence. Account authorization, provider acceptance and remote presentation are separate.
After answer, preserve SDP, selected codec, payload mapping, RTP tuples, packet direction and decoding. The published codec list does not prove negotiation, and a SIP dialog does not prove audible two-way media.
Retest one correction and keep continuity evidence bounded
Apply one authorized correction, then repeat the affected ordinary call and an unaffected control. Preserve registration, inbound or outbound signaling, route match, answer, both media directions and hangup. If Offline diversion is configured, record the trigger, destination, completed alternate call and restoration separately.
Use outage notices only as timestamped service-wide context. Port completion does not prove PBX readiness, and 000 must not be used as an uncoordinated test. Verify IPND address, presented identity, callback, power, internet and an independent calling method through approved procedures. No trunk, call, diversion, media or emergency result was tested for this diagnostic.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- MaxoTel — 3CX SIP Trunking, last modified 24 May 2023 ↗
- MaxoTel — Portal Guide: PBX Extensions and SIP Trunks, last modified 11 August 2023 ↗
- MaxoTel — 3CX Suddenly Loses SIP Trunk Registration, last modified 21 April 2020 ↗
- MaxoTel — SIP ALG, last modified 19 June 2019 ↗
- MaxoTel — NAT Issues, last modified 20 April 2021 ↗
- MaxoTel — Which codecs are supported?, last modified 19 April 2021 ↗
- MaxoTel — What VoIP Protocols are supported? ↗
- MaxoTel — Outage Communications Policy ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.