Fax Without the Legacy Hardware
DIDLive supports inbound fax over SIP using the T.38 protocol, allowing you to receive faxes on your existing DID numbers without dedicated fax lines.
What is Fax over SIP?
Fax over IP (FoIP) is a technology that enables fax transmissions over the internet instead of traditional analog phone lines. By converting fax data into digital packets, it enables reliable document transmission across IP networks. For industries like healthcare, legal, finance, insurance, and manufacturing, faxing remains a required method for secure real-time document transmission.
The critical technology behind reliable FoIP is the T.38 protocol — an ITU-T recommendation specifically designed for transmitting fax data over IP networks. Unlike voice codecs such as G.711 which are optimized for speech, T.38 is purpose-built for fax and handles the unique requirements of image data transmission including packet redundancy and error correction.
T.38 vs. G.711 for Fax
Voice calls can tolerate small amounts of packet loss — a few missing audio frames are barely noticeable to the human ear. Fax transmissions, however, cannot tolerate any packet loss because missing data means corrupted document images. This is precisely why a specialized protocol like T.38 is needed.
T.38 (Recommended)
Designed specifically for fax. Converts fax tones into data packets with built-in redundancy for both control and data. Can tolerate up to 1 second of delay but requires zero packet loss. Supports ECM error correction for perfect image reproduction.
G.711 (Fallback)
Passes fax audio as voice. Works over standard RTP streams but is unreliable for fax because voice codecs don't support the packet redundancy that fax data requires. Used as a fallback when T.38 is not available on the call path.
How It Works
Fax calls over SIP typically begin as standard audio sessions using G.711. When fax tones (CNG/CED) are detected on the call, the SIP endpoint sends a re-INVITE requesting a switchover to T.38 UDPTL. Once negotiated, the fax data is transmitted as a digital data stream rather than as audio, providing reliable delivery.
For this to work properly, T.38 support must be present throughout the entire call path — from the fax device, through any intermediate PBX or session border controller, to the SIP trunk provider. DIDLive supports T.38 on our SIP trunks, ensuring the protocol negotiation happens correctly for reliable fax transmission.
Whether you're connecting a traditional fax machine through an ATA (Analog Telephone Adapter), using a fax server, or running a softfax application, DIDLive's T.38-capable SIP trunks provide the reliable foundation your fax communications need.