Fax support#

DIDWW delivers inbound fax over SIP trunks using T.38 fax relay. Fax calls reach a compatible fax server, gateway, PBX, or fax endpoint through the same inbound SIP routing used for voice calls.

T.38 is designed specifically for fax transport over IP networks. It requires a DID number with the Inbound Fax feature and a receiving system that negotiates T.38 during the call.

This page describes fax delivery to customer equipment through an inbound SIP trunk. For fax delivery to email or cloud storage through phone.systems™, see the DIDWW Fax service.

How fax delivery works#

An incoming fax follows the inbound routing configured for its DID number. DIDWW delivers the call to the assigned SIP trunk, and T.38 is negotiated with the receiving system during the call.

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flowchart LR
    SENDER["Sending fax device"]
    DID["DIDWW DID number with<br/>Inbound Fax feature"]
    TRUNK["Inbound SIP trunk"]
    T38["T.38 negotiated<br/>with receiving system"]
    RECEIVER["Customer fax system"]

    SENDER --> DID --> TRUNK --> T38 --> RECEIVER

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    classDef process fill:#ccfbf1,stroke:#2dd4bf,color:#1f2d3d,stroke-width:1.5px

    class SENDER,RECEIVER endpoint
    class DID,TRUNK,T38 process

    linkStyle default stroke:#0066cc,stroke-width:1.5px

T.38 carries fax data using a protocol designed for fax over IP. It is more resilient to network conditions than transporting fax tones as audio and supports error correction. SIP signaling establishes and ends the call as for a voice call.

Note

G.711 audio can act as a failover when T.38 cannot be negotiated. Keep a G.711 codec (PCMU or PCMA) enabled on the trunk and the receiving system for this purpose. Fax over G.711 is best effort and is not guaranteed to succeed.

Requirements#

  • A DID number with the Inbound Fax feature.

  • An inbound SIP trunk assigned to that number.

  • A receiving system configured to accept T.38.

  • SIP signaling and media traffic allowed between DIDWW and the receiving system. See the Firewall and SBC allowlist.

Availability#

T.38 is available for DID numbers with the Inbound Fax feature. Availability depends on the country, area, and number type. To find eligible numbers, use the Inbound Fax filter under Supported feature filters. The Fax service coverage page lists the countries where fax is supported.

Troubleshooting#

The fax call does not reach the receiving system.

Confirm that the DID number is active and assigned to the intended inbound SIP trunk. Verify the trunk destination and SIP signaling connectivity.

T.38 is not negotiated.

Confirm that the DID number has the Inbound Fax feature and that the receiving system is configured to accept T.38.

Fax transmission is incomplete or unreliable.

Confirm that the call used T.38. If the call fell back to G.711 audio, delivery is best effort.

Check the media path for packet loss, jitter, blocked ports, or NAT issues.

For call-specific signaling results, review SIP response codes in Call Logs and the related call records.

Standards#

  • ITU-T T.38 defines real-time Group 3 fax communication over IP networks.