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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.

Reviewed 2026-10-15Analog interface and carrier behavior vary by device, region and firmwareIntermediateDocumentation based · not lab tested

Two different transport approaches

G.711 passthrough carries the fax tones through an audio path. T.38 defines real-time Group 3 facsimile communication over IP, allowing compatible gateways to relay fax information rather than treating the entire exchange as ordinary speech audio. An ATA advertising T.38 does not establish support across the PBX, carrier and remote gateway. A path can begin as audio and attempt to switch after fax detection, so inspect the actual session negotiation.

Verify the transition and fallback

Look for the relevant session update and its accepted media description. If the T.38 request is rejected, determine whether a supported G.711 fallback occurs or the call fails. Do not infer that a selected GUI preference became the active transport. For passthrough, avoid unexpected lossy transcoding and investigate packet loss, timing and media processing that can disturb modem tones. Device guidance may recommend specific settings; apply them to the exact firmware and test rather than treating one generic fax recipe as universal.

Test documents, not just tones

Send and receive representative multi-page documents with the intended remote destinations. Record page count, negotiated mode, completion result and any retry. Hearing fax tones or receiving the first page does not establish reliable service. Keep error-correction and speed changes evidence-driven. Reducing speed can help isolate a problem but may hide an unreliable path. For critical document workflows, compare a supported fax service or another agreed delivery method with maintaining the analog machine. Choose based on measured delivery needs and operating responsibility.

Read the mode selected by an Asterisk fax session

The Asterisk res_fax guide keeps T.38 negotiation, allowed or forced audio behavior and the reported FAXMODE independently observable instead of treating capability as use.

Inspect a documented RTP-to-UDPTL transition

Use the T38Fax negotiation diagnostic to correlate the initial G.711 phase, T.38 re-INVITE, SDP answer, UDPTL packets, ECM and document result.

Separate Clearfly managed fax from trunk fax transport

Clearfly lists fax-to-email or eFax-to-email. The provider record does not convert that managed product into evidence for T.38, audio pass-through, an ATA or an existing fax machine.

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

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