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Fax over IP: T.38 and audio passthrough
Fax modems are sensitive to delay, jitter, packet loss, compression, and echo cancellation. A VoIP voice call can sound acceptable while a fax fails because fax signalling needs a more stable media path.
Two common approaches
| Approach | Description |
|---|---|
| T.38 | Converts the fax modem exchange into a packet-oriented relay |
| G.711 passthrough | Carries modem audio without lossy compression or aggressive voice processing |
Support must exist end-to-end: endpoint, PBX, SBC, carrier, and destination. Compare the negotiated SDP, rate management, packet loss, and whether a gateway changed the media mode mid-call.
For important fax services, test real documents and failure recovery rather than relying only on a successful short page.
Related: voice codecs, RTP ranges, and QoS.