T38 fax is a protocol that enables faxing over IP networks. It converts the analog signals of a fax machine into digital packets that can be transmitted reliably across a VoIP network. Read our How does fax work in VoIP environments? article for more information. While voice calls can tolerate small drops and jitter, fax transmissions can’t. They depend on a stable, continuous signal. t38 fax solves this by encapsulating the fax data, handling packet loss, and maintaining timing.

Sending a fax across a data network requires a different approach than voice. VoIP compresses and chops up audio. That’s fine for conversations, but it breaks fax signals. T.38 fax intercepts the fax signal and repackages it into IP-friendly data. The result is a system that allows businesses to send and receive faxes without relying on old analog phone lines.

T.38 is described in RFC 3362, and defines how a device should communicate the fax data. In the diagram above both the gateway and the fax machine behind the gateway would have to be T.38-capable. For the G3 fax machine on an analog line, this process will be transparent. The analog fax machine does not need to know T.38.

This makes T.38 essential for any business that has moved to a VoIP phone system but still needs to send or receive faxes. Without it, faxing over VoIP is unreliable and usually fails.

T38 fax works by acting as a middle layer between a traditional fax machine and the IP network. It captures the data from the fax and sends it as image-based packets across the internet or private IP infrastructure.

The basic process is:

• A fax machine or T.30 device initiates a transmission.
• A VoIP gateway intercepts and converts the analog signal into T.38 format.
• The T.38 stream travels across the IP network using either TCP or UDP.
• At the destination, the stream is converted back to a format the receiving device understands.

This setup can occur in different ways. The most common is a fax relay. That’s when a T.30 fax machine sends a document to a T.38-capable VoIP gateway. That gateway takes the analog signal and converts it to digital. It then sends that stream to another gateway, server, or T38-enabled fax machine.

A more modern version of this is an Internet-Aware Fax device. This type of fax machine or software is already built to handle T38 directly. It can send and receive fax data over the IP network without ever touching the PSTN.

Faxing is still used in industries like healthcare, law, education, and public services. But many of these organizations have moved their phones to VoIP. When that happens, analog fax machines no longer work reliably over VoIP lines. Fax tones don’t survive compression and packet loss. The result is failed transmissions, missing pages, or corrupted documents.

T.38 fax bridges the gap between old fax technology and modern IP networks. It allows organizations to keep their fax workflows while removing the cost and maintenance of analog phone lines.

Using T.38 removes the need for fallback solutions like G.711 passthrough. G.711 sends the audio signal directly over IP as if it were a voice call. This works poorly for fax. Any jitter or delay can cause failure. T.38 solves that by using a different method altogether – one built specifically for fax data.

T.38 fax delivers several key advantages for businesses that still rely on faxing. These include:

1. Improved reliability over VoIP
Fax signals are sensitive to timing and packet loss. T.38 compensates for these issues, resulting in more successful fax transmissions.

2. Lower costs
Fax over IP means no more dedicated analog lines. That reduces monthly phone bills and removes the need to maintain legacy infrastructure.

3. Easier document delivery
When integrated with a fax server like 3CX, incoming faxes are automatically converted to PDF and sent via email. No paper, no printing, no maintenance.

4. Full digital record keeping
Faxes received through T.38 are saved as digital documents. This makes them easier to archive, search, and secure.

5. Remote access
Employees working remotely can still receive faxes. Since the documents arrive via email, there’s no need to be physically near a fax machine.

6. Works with existing fax hardware
There’s no need to buy new fax machines. With a compatible ATA or VoIP gateway, your current hardware can work with T.38.

These benefits are what make T.38 fax a practical and cost-effective solution for businesses transitioning from traditional phone systems to modern VoIP systems.

T.38 fax was built with IP networks in mind. It takes advantage of several features that help maintain reliability and quality.

One of the most important is the use of UDP for packet delivery. Unlike TCP, which waits for confirmation of every packet, UDP simply sends the data. This prevents delays caused by retransmissions, which can break the timing requirements of fax transmissions.

Another important feature is redundancy. T.38 can include duplicate packets to account for potential packet loss. If one packet is lost, the redundant copy fills in the gap. This helps maintain data integrity and ensures the complete fax is received.

T.38 also defines how devices should negotiate and handle errors during a session. These features make it suitable for use over unpredictable or low-quality internet connections. By using packet-based data instead of analog audio, T.38 reduces the risk of failed faxes.

While T.38 fax is designed for IP networks, it still works alongside the PSTN. In fact, many fax transmissions involve both. A T.30 fax machine may send a fax over a PSTN line to a VoIP gateway. That gateway then converts the signal to T.38 and sends it over IP. On the other end, a receiving gateway converts it back and passes it to another fax machine on the PSTN.

This hybrid setup allows businesses to communicate with partners, suppliers, and customers who still use traditional phone lines. It ensures compatibility while moving most of the fax infrastructure to IP.

Over time, businesses can phase out PSTN completely by adopting fax servers or internet-aware fax devices. These options use T.38 end to end, avoiding the need for any analog lines.

3CX includes a built-in T.38 fax server. This means you can receive faxes from anywhere in the network and have them automatically forwarded to your email inbox as PDF files.

Here’s how it works:

• An external fax device or service sends a fax to your VoIP number.
• The SIP trunk receives the call and passes it to 3CX.
• 3CX recognizes the T.38 stream and converts the fax to a digital document.
• The PDF is then emailed to the intended recipient.

This setup doesn’t require physical fax machines or paper. It’s a full fax-to-email solution built into your PBX system.

If you need to send faxes, you can still do so by using an external fax machine connected via a T.38-capable gateway or ATA. Many businesses find that receiving by email is enough and stop sending faxes altogether, opting instead for secure document uploads or email.

Faxing may be seen as outdated, but many industries still rely on it for legal or compliance reasons. T.38 fax brings this old technology into the modern era by making it work over VoIP and IP networks. It allows businesses to maintain legacy workflows without the cost or complexity of analog phone systems.

By removing the need for paper, phone lines, and physical devices, fax over IP supports modern work habits. Employees can work remotely, access their documents from anywhere, and avoid the delays associated with physical fax machines.

Fax Over IP (FoIP): The method of sending faxes using an internet protocol network instead of traditional phone lines.

T.30: The protocol used by traditional fax machines over analog lines.

T.38: The protocol that enables fax transmission over IP.

VoIP: Voice over IP. A technology that sends phone calls over data networks.

ATA: Analog Telephone Adapter. A device that allows analog phones or fax machines to connect to VoIP networks.

PSTN: Public Switched Telephone Network. The old-school phone system used for analog voice and fax communication.

Businesses moving to VoIP often find that faxing becomes unreliable. That’s because fax wasn’t built for the way VoIP works. T.38 fixes this and provides a reliable bridge between analog faxing and digital networks. It reduces costs, improves reliability, and simplifies document handling.

With faxing still required in many sectors, T.38 fax ensures it can remain part of the workflow without holding your communications system back.