Faxing over SIP

Cloud Voice supports both sending and receiving faxes. Inbound faxing works by routing a phone number to the fax service; outbound faxes can be submitted programmatically through the API. Fax devices are managed through the fax service rather than as regular SIP trunks, and every transmission reports its progress through status events.

Receiving faxes

Route a phone number to fax from the number’s detail panel on the Numbers page in the dashboard, the same panel where you set trunk, SIP URI, and other routing (see Inbound Number Routing). Calls to that number are then answered by the fax service and received as documents.

Viewing received faxes

Detailed steps coming soon.

Sending faxes

Outbound fax transmission is available through the API, you submit a document and a destination number, and the platform originates the fax call and reports the result. See the API Reference.

Sending from the dashboard

Detailed steps coming soon.

Transmission status

Fax transmissions are tracked end to end, including the negotiation with the far-end machine, and status updates are delivered as events, so your systems can know whether a fax succeeded, failed, or needs to be retried. See Webhooks & Events.

Reliability guidance

Fax is far less tolerant of network imperfections than voice: a moment of packet loss that a caller wouldn’t notice can abort a fax page. To keep success rates high:

• Use uncompressed audio codecs (PCMU/PCMA) for any trunk path that carries fax, compressed codecs such as G729 corrupt fax tones.
• Keep the network path stable: jitter and packet loss are the leading causes of mid-page failures; prefer wired, uncongested links.
• Prefer the platform fax service over running fax through your own PBX audio path, the service handles the fax protocol directly and tracks negotiation results.
• Watch the status events: a failure report includes the stage at which the transmission failed, which tells you whether to blame the line, the far end, or the document.

Note

Multi-page and high-resolution documents take proportionally longer and are more exposed to line quality; if a long fax fails repeatedly, try splitting it.