# 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(TM), see
the [DIDWW Fax service](https://www.didww.com/services/fax).

## 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.

```
        %%{init: {
  "theme": "base",
  "themeVariables": {
    "fontSize": "14px",
    "primaryColor": "#e0f2fe",
    "primaryBorderColor": "#38bdf8",
    "primaryTextColor": "#1f2d3d",
    "secondaryColor": "#ccfbf1",
    "secondaryBorderColor": "#2dd4bf",
    "secondaryTextColor": "#1f2d3d",
    "tertiaryColor": "#fef3c7",
    "tertiaryBorderColor": "#facc15",
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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

    classDef endpoint fill:#e0f2fe,stroke:#0066cc,color:#1f2d3d,stroke-width:1.5px
    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](../sip-connectivity-reference.html#inbound-sip-connectivity-firewall).

## 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](../../../../phone-numbers/buy-numbers/filters-reference.html#user-panel-buy-numbers-filters). The [Fax service
coverage](https://www.didww.com/services/fax) 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](../../analytics/sip-response-codes-reference.html#inbound-technical-sip-response-codes-in-call-logs) and the
related call
records.

## Standards

- [ITU-T T.38](https://www.itu.int/rec/T-REC-T.38) defines real-time Group 3
  fax communication over IP networks.

## Related resources

- [Create a SIP trunk](../how-to-guides/create-sip-trunk.html) - Configure
  the inbound SIP destination that receives fax calls.
- [SIP trunk settings reference](../sip-trunk-settings-reference.html) -
  Review the fields and available values for inbound SIP trunks.
- [SIP connectivity reference](../sip-connectivity-reference.html) - Review
  DIDWW signaling addresses, media networks, ports, and codecs.
- [Assign a voice trunk](../../../../phone-numbers/my-numbers/how-to-guides/assign-voice-trunk.html#user-panel-assign-trunk) - Assign the inbound
  SIP trunk to one or more DID numbers.
- [Fax service coverage](https://www.didww.com/services/fax) - Check the
  countries where fax is supported.

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