Fax Over IP: Why Faxing Over VoIP Is Unreliable and What to Do About It

When Your Phones Sound Bad: Part 12 of 12

Every other post in this series has been about voice calls. This one is about faxing, which might seem out of place in a VoIP troubleshooting guide, but it's here for a good reason: fax problems are one of the most common complaints from businesses that have moved to VoIP. The phones work fine. The fax machine doesn't. It drops pages, fails to connect, sends garbled documents, or just refuses to work at all.

The frustrating part is that this isn't really a bug. Fax machines were designed for a specific kind of network, and VoIP is a fundamentally different kind of network. Understanding why faxing struggles on VoIP explains the problem and, more importantly, helps you pick the right solution.

Why fax and VoIP don't mix

Traditional fax machines communicate by sending modulated audio tones over a phone line. One machine calls the other, they negotiate a speed and encoding using a series of handshake tones, and then the sending machine converts the document into audio signals that the receiving machine decodes back into an image. The entire process relies on the audio connection being clean, consistent, and uninterrupted.

On a traditional phone line (POTS, Plain Old Telephone Service), this works well because the audio path is a dedicated circuit with predictable, consistent characteristics. The signal goes in one end and comes out the other end essentially unchanged.

VoIP doesn't work this way. VoIP takes audio, encodes it with a codec, breaks it into packets, sends those packets across the internet, and reassembles them at the other end. This process introduces several things that fax machines can't tolerate.

Compression. Most voice codecs are lossy, meaning they discard audio information that humans won't notice to reduce bandwidth. G.729, Opus, and other compressed codecs do this aggressively. The audio sounds fine to a human ear because the discarded information falls outside the range that matters for speech comprehension. But a fax machine isn't listening to speech. It's listening to precise tonal signals, and the information that the codec discarded might include critical parts of those signals. A compressed codec can make fax tones unrecognizable to the receiving machine.

Jitter. Fax machines expect tones to arrive at precise, consistent intervals. The jitter buffer in a VoIP system introduces variable delay that shifts the timing of the received audio. For voice, this is fine because human ears are tolerant of small timing variations. For fax, the timing variations can cause the receiving machine to misinterpret the signals.

Packet loss. Even a tiny amount of packet loss, which would be barely noticeable on a voice call, can corrupt a fax transmission. A lost packet means a gap in the tone sequence, which the receiving fax machine may interpret as a different signal, a transmission error, or a disconnect command. Human ears fill in small gaps in speech automatically. Fax machines don't.

Echo cancellation. VoIP systems typically run echo cancellation on the audio path to prevent voice echo. Echo cancellers can interfere with fax tones because they may treat the fax signals as echo and try to remove them. Most VoIP systems are supposed to disable echo cancellation when they detect fax tones, but this detection doesn't always work reliably.

G.711 passthrough: the minimum viable approach

The simplest approach to faxing over VoIP is to ensure that the fax connection uses G.711 (either PCMU or PCMA) without any compression, echo cancellation, or silence suppression. G.711 is an uncompressed codec that passes audio through without modification. If the fax tones go in one end as G.711 and come out the other end as G.711, with no transcoding in between, the tones arrive essentially unchanged.

For this to work, every component in the path between the two fax machines needs to pass G.711 without modification. Your PBX needs to use G.711 on the ATA port connected to the fax machine. Your SIP trunk needs to use G.711 for the fax call. No transcoding can happen anywhere in the chain. Echo cancellation and silence suppression need to be disabled for the fax connection.

G.711 passthrough works reasonably well under good network conditions. If your internet connection has low latency, low jitter, and minimal packet loss, faxes will often transmit successfully. But "reasonably well" and "reliably" are different things. Even with G.711, the packetization process introduces jitter that traditional fax wasn't designed for, and any network hiccup during the transmission can corrupt or fail the fax.

Many businesses use G.711 passthrough for faxing and find it works well enough most of the time. If your faxing volume is low and the occasional failed fax isn't a serious problem, this approach may be sufficient.

T.38: the proper solution for fax over IP

T.38 is a protocol specifically designed for fax over IP. Instead of trying to carry fax tones as audio over a voice codec (which is what G.711 passthrough does), T.38 decodes the fax signals at the sending end, converts them into data packets (IFP, Internet Facsimile Protocol packets), sends those packets over the IP network, and then reconstructs the fax signals at the receiving end.

The critical difference is that T.38 sends the fax data as data, not as audio. The tonal signals are decoded into their meaning (image data, handshake commands, timing signals) and transmitted digitally. This eliminates the sensitivity to codec compression, jitter, and the other audio path issues that make fax unreliable on VoIP.

T.38 also includes error correction mechanisms. In its typical deployment over UDPTL (UDP Transport Layer for T.38), it uses forward error correction through redundancy: each UDPTL packet includes copies of previous IFP packets, so if a packet is lost, the data it carried is likely included in a subsequent packet. This is not retransmission in the TCP sense. The sender does not wait for acknowledgment and resend lost packets. Instead, the redundancy is sent proactively so the receiver can reconstruct missing data from later packets. (T.38 can also run over TCP, which would provide true retransmission, but TCP-based T.38 is rarely used in practice.) This redundancy mechanism makes T.38 significantly more reliable than audio passthrough on networks with moderate packet loss.

For T.38 to work, both your side and the remote side of the fax connection need to support it. Specifically:

Your ATA (analog telephone adapter, the device that connects your fax machine to the VoIP network) needs to support T.38. Most modern ATAs do.

Your PBX needs to support T.38 and be configured to use it for fax calls. Asterisk, FreeSWITCH, and most commercial PBX platforms support T.38, but it often needs to be explicitly enabled.

Your SIP trunk provider needs to support T.38 on their side. Not all providers do, and among those that do, the quality of their T.38 implementation varies. Ask your trunk provider whether they support T.38 and whether there's any additional configuration needed on their end.

When a fax call is placed over a T.38 capable connection, the call typically starts as a normal voice call using G.711. When the system detects fax tones (the initial CNG or CED tones that fax machines exchange), it switches the call from audio mode to T.38 mode. This switchover is called a "T.38 reinvite" because the system sends a SIP re-INVITE requesting to change the media type from audio to T.38.

If the switchover fails (because one side doesn't support T.38, or the reinvite is blocked by a firewall or SBC), the call may fall back to G.711 passthrough or may fail entirely, depending on the configuration. Monitoring the SIP reinvites during fax calls is one way to diagnose T.38 issues.

Fax over email (the practical alternative)

For many businesses, the most reliable solution to fax over VoIP problems is to stop sending fax over VoIP entirely and switch to a fax over email service instead.

Fax over email services (sometimes called electronic fax or internet fax) work by receiving faxes on the provider's infrastructure (which has dedicated, optimized fax connections) and delivering them to you as email attachments, typically PDF files. Outbound faxes are sent by emailing a document to the service, which transmits it from their infrastructure.

The advantages are significant. You don't need a fax machine. You don't need an ATA. You don't need to worry about T.38 support, codec configuration, or network quality affecting fax reliability. The provider handles all the tricky parts on infrastructure designed for faxing, and you interact with faxes as simple email attachments.

Many providers offer fax over email services, some bundled with VoIP phone service and some as standalone products. For businesses that still need to send and receive faxes but are struggling with reliability over their VoIP connection, this is often the most practical solution.

Keeping a traditional fax line

For businesses with high fax volume or regulatory requirements around faxing (healthcare and legal are the most common), keeping a traditional analog phone line specifically for the fax machine is worth considering. A single POTS line dedicated to faxing eliminates all VoIP related fax issues because the fax traffic never touches the VoIP system.

The cost of a single analog line is modest, typically $20 to $40 per month depending on your area. Compared to the time and frustration of troubleshooting intermittent fax failures over VoIP, a dedicated fax line can be a bargain.

This approach is particularly appropriate when fax reliability is critical, when you transmit long or complex documents (which are more susceptible to errors over VoIP), or when the other parties you fax with regularly are on systems that don't support T.38. One caveat: AT&T begins decommissioning its copper network in mid-2026, and other carriers have their own retirement timelines, so treat a dedicated analog fax line as a bridge rather than a permanent solution.

When fax over VoIP fails intermittently

If you're using G.711 passthrough or T.38 and faxes work sometimes but fail other times, the network conditions during the failure periods are almost certainly the cause. Fax is far less tolerant of network imperfections than voice. A level of jitter or packet loss that's completely invisible on a voice call can be enough to fail a fax.

Run network quality tests during the times when faxes are failing. If you see any packet loss above 0.1% or jitter above 20 milliseconds, those conditions are likely causing the failures. Improving the network conditions (QoS, bandwidth management, connection upgrade) will help, but fax will always be the first thing to break when network quality degrades because it has the lowest tolerance of any application running over VoIP.

Reducing the fax speed can also help. Most ATAs allow you to configure the maximum fax speed. The default is often 14400 bps. Reducing it to 9600 bps gives the fax protocol more time to recover from errors and makes the transmission more tolerant of network imperfections. You'll trade speed for reliability, but most faxes at 9600 bps still complete in a reasonable time.

This is the final post in the When Your Phones Sound Bad series. For foundational VoIP knowledge, the VoIP From the Ground Up series starts from the basics. For internet connection guidance, the Getting the Internet Right series covers selecting and evaluating business internet. For protocol level troubleshooting, the SIP Signaling From the Wire Up series goes deep on the signaling layer.