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Fax-over-VoIP troubleshooting by failure stage

Distinguish call setup, fax negotiation, page transfer and completion failures.

Reviewed 2026-10-13Analog interface and carrier behavior vary by device, region and firmwareIntermediateDocumentation based · not lab tested

Name the stage that failed

Record the sending and receiving equipment, direction, page count, time and displayed error. Check whether the call reached the destination, whether fax negotiation began, whether any pages transferred and whether both sides reported completion. Repeat with a known test document and a known destination. A failure with one remote machine suggests a different investigation from failures with every destination. Avoid changing codec, speed and error correction together before retaining a baseline.

Inspect the actual media mode

Capture authorized signaling around fax detection and any re-INVITE. Determine whether T.38 was offered and accepted, rejected with fallback, or never attempted. For G.711 passthrough, confirm that the full path stays compatible and does not introduce an unexpected codec conversion. Compare RTP or relay-path loss and timing with the failure interval. A successful voice call does not exclude impairments that disrupt fax. Also inspect local analog cabling, ring behavior and ATA regional settings before attributing every failed page to the provider.

Validate the recovery

After one bounded change, test both sending and receiving, short and multi-page documents, and the remote destinations relevant to the business. Record completion at both ends where possible. A sender's apparent success and an unreadable received document are not equivalent to successful delivery. Retain only necessary diagnostic evidence. Fax content can be sensitive; use a non-sensitive test page and redact identifiers before sharing traces. If the supported path remains unreliable, escalate with exact timestamps, negotiation results and device versions instead of repeatedly resetting the ATA.

Move to FreePBX delivery only after session success

When Asterisk reports a completed fax session but FreePBX email, attachment or UCP state is missing, use the FreePBX post-receipt diagnostic. It keeps media negotiation separate from recipient mapping and distribution.

Use the Asterisk branch for a terminating or originating fax application

When Asterisk runs ReceiveFAX() or SendFAX(), use the focused res_fax workflow to prove modules, session options, file access, media mode and the complete result tuple.

Separate call setup, T.38 negotiation and document acceptance

The T38Fax symptom workflow preserves each evidence stage instead of treating an answered SIP call as a successful fax.

Sources & applicability

Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.

Examples require adaptation to your topology. No live PBX or hardware testing is claimed.

Continue your research

Guide · Analog interface and carrier behavior vary by device, region and firmware

Fax over VoIP: T.38 relay versus G.711 passthrough

Choose a fax path from end-to-end support and observed document delivery, not a codec checkbox.

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Guide · Wireshark 4.x / current upstream tool documentation; inspect installed versions

Find RTP streams and interpret loss and jitter

Separate captured packet statistics from what the receiving listener actually experienced.

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Guide · Wireshark 4.x / current upstream tool documentation; inspect installed versions

Correlating SIP call legs with PBX events

Follow transfers and bridges without assuming one Call-ID represents the entire business call.

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Guide · FreePBX 17 / Debian 12 editorial baseline; Sangoma Fax Pro documentation and product boundary, FreePBX Fax Configuration release/17.0 source, and current Asterisk ReceiveFAX, SendFAX and FAXOPT documentation reviewed October 11, 2026; the Sangoma guide includes historical interface wording and installed module, license, generated dialplan and live behavior remain authoritative; no PBX, license, DID, route, fax user, extension, trunk, fax session, file, email, UCP item, storage quota, document or remote machine was inspected or tested

FreePBX fax: inbound routing, storage and delivery evidence

Separate DID routing, fax transfer, recipient ownership, local storage, UCP visibility and email delivery so each stage can be accepted and diagnosed.

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Troubleshooting · FreePBX 17 / Debian 12 editorial baseline; Sangoma Fax Pro documentation and product boundary, FreePBX Fax Configuration release/17.0 source, and current Asterisk ReceiveFAX and FAXOPT documentation reviewed October 11, 2026; installed module, license, generated dialplan, storage, mail transport and live behavior remain authoritative, and no PBX, license, DID, route, recipient, fax session, file, conversion, message, mailbox, UCP item, quota, document or remote machine was inspected or tested

FreePBX received the fax but email or the UCP copy is missing

Start with one completed ReceiveFAX session, then trace recipient mapping, attachment conversion, local storage, quota, UCP visibility and email delivery.

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Guide · Asterisk 22 FAXOPT, ReceiveFAX and SendFAX documentation plus branch 22 res_fax configuration and implementation reviewed October 13, 2026; the installed Asterisk version, res_fax module, technology resource, configuration, channel, TIFF/F file, signaling, T.38 negotiation, audio path, session result, storage and delivery remain deployment specific, and no host, module, file, channel, packet, fax session or document was inspected or tested

Asterisk res_fax: session options, mode and result evidence

Keep the generic res_fax layer, loaded technology resource, per-session FAXOPT policy, TIFF/F file, audio or T.38 mode and final transmission result independently observable.

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Troubleshooting · Asterisk 22 FAXOPT, ReceiveFAX and SendFAX documentation plus branch 22 res_fax configuration and implementation reviewed October 13, 2026; the installed Asterisk version, res_fax module, technology resource, configuration, channel, TIFF/F file, signaling, T.38 negotiation, audio path, session result, storage and delivery remain deployment specific, and no host, module, file, channel, packet, fax session or document was inspected or tested

Asterisk ReceiveFAX or SendFAX fails or uses the wrong mode

Trace one fax from the exact channel and loaded res_fax stack through FAXOPT inputs, file access, audio or T.38 negotiation, application result and downstream artifact handling.

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Troubleshooting · T38Fax testing, firewall, SIP ALG, packet-capture, ECM and fax-only service pages reviewed October 13, 2026; the exact trunk, endpoint, intermediary, authentication, registration, SIP dialog, re-INVITE owner, SDP, NAT, firewall, ALG, UDPTL packets, fax speed, resolution, ECM result, duration and received document remain deployment specific, and no account, endpoint, PBX, SBC, firewall, packet, call, fax session or document was inspected or tested

T38Fax call connects but T.38 is not negotiated

Trace one fax from the initial SIP dialog and G.711 phase through the T.38 re-INVITE, SDP answer, UDPTL path, ECM state and received-document result.

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