Fax Over VoIP Slow? Why It Happens and How to Fix It
Fax over VoIP is slow because voice networks distort the fax modem signal, so the machine steps down from 14,400 bps to 9,600, 4,800 or 2,400 bps. Lower the fax speed, keep ECM on, and use G.711 or T.38 on the ATA.
- A clean page at 14,400 bps takes roughly 20 to 60 seconds, so several minutes per page means speed fallback or retransmission.
- On Brother machines, set Compatibility to Basic (for VoIP); it disables V.34 Super G3 and caps speed at 14,400 bps.
- Keep ECM on so errors are retransmitted instead of printed as garbled lines, accepting some extra time on a poor line.
- The ATA fax port needs G.711 only, VAD off, echo cancellation off, call waiting off and a fixed jitter buffer, or T.38 if both ends support it.
- If the fixes fail after testing with a second number, a hosted online fax service or an analog line is more reliable than more tuning.
Why is fax so slow on VoIP?
A fax machine does not send digital data. It sends audio tones using the T.30 protocol with modulations such as V.27ter (2,400 and 4,800 bps), V.29 (7,200 and 9,600 bps) and V.17 (up to 14,400 bps), and V.34 on Super G3 machines (up to 33,600 bps). VoIP was designed to carry human speech, so it compresses, smooths and edits audio in ways that damage these tones.
Packet loss removes slices of the signal, jitter makes timing wobble, and compression codecs such as G.729 discard exactly the detail a modem needs. Echo cancellers and silence suppression (VAD) also treat modem tones as noise and clip them.
The fax machine cannot tell which of these is happening. It only sees bad frames during training, so it falls back to a slower, more robust modulation and tries again. That fallback is the slowness you see.
How long should a fax page take?
Before changing anything, work out whether your fax is slow or just normal. A text-only page in standard resolution at a clean 14,400 bps takes about 20 to 60 seconds, plus 10 to 30 seconds of call setup and handshake for the whole transmission.
| Negotiated speed | Typical time per text page | What it usually means |
|---|---|---|
| 33,600 / 14,400 bps | 20 to 60 seconds | Healthy link |
| 9,600 bps | about 45 to 90 seconds | Mild impairment, acceptable |
| 4,800 bps | about 1.5 to 3 minutes | Noticeable loss or jitter |
| 2,400 bps | about 3 to 5 minutes | Severe fallback, fix needed |
| Repeated retries, then error | Never completes | Loss, codec or ATA problem |
Which fax machine settings should I change first?
These are free and take minutes, so do them before touching the ATA or calling the provider. Work in this order and send one test fax after each change so you know which one helped.
- Brother: press Menu > Initial Setup > Compatibility (some models: Menu > Fax > Setup > Compatibility) and choose Basic (for VoIP). This disables V.34 Super G3 mode and limits speed to 14,400 bps. If Basic is not enough, try Normal, which also trims the modem speed.
- Other brands: look for a modem or transmission speed setting. HP uses Fax Speed with Fast (V.34), Medium (V.17) and Slow (V.29). Epson uses Fax Speed with Fast, Moderate and Slow. Labels vary by model, so choose the option that caps at 14,400 or 9,600 bps.
- If a lower cap still fails, set the start speed manually to 9,600 bps, then 4,800 bps. Starting low avoids repeated failed training attempts at high speed.
- Keep ECM (Error Correction Mode) on. ECM sends the page in blocks and retransmits bad ones, so errors cost time instead of producing streaky or missing lines. On a very poor line ECM can add noticeable time, but turning it off usually trades speed for corrupted pages.
- Send at standard resolution, not fine or photo, and use a plain text page for testing.
What ATA and codec settings does a fax port need?
The ATA (the modem or adapter supplied by the provider, or your own) sits between the analog fax and the SIP network, so its fax port settings matter most. Many carrier-supplied units hide these in a web interface or only allow the provider to change them. If you cannot reach them, ask support for the same changes.
The goal is to make the audio path as transparent as possible, or to hand the fax off to T.38.
| Setting | Fax value | Why |
|---|---|---|
| Codec | G.711 only (u-law in North America, a-law elsewhere) | No lossy compression; G.729 and similar break modem tones |
| VAD / silence suppression | Off | Prevents clipped tones and dropouts during pauses |
| Echo cancellation (G.168) | Off on the fax port | Cancellers distort or suppress modem signals |
| Call waiting | Off | A beep mid-page can abort the call |
| Jitter buffer | Fixed, not adaptive | Adaptive buffers change delay mid-call and upset modem timing |
| Fax mode / T.38 | Enable T.38, or select the FAX profile | Switches the unit to fax-aware handling |
| Packetization | 20 ms | Standard, keeps loss impact small |
T.38 or G.711 passthrough: which is better for fax?
G.711 passthrough sends the fax tones as ordinary audio. It works when the path is clean, because G.711 is uncompressed, but every lost packet is lost signal.
T.38 demodulates the fax at one end, sends the data as UDPTL packets with built-in redundancy or forward error correction, and re-modulates it at the other end. It tolerates loss and jitter far better, which is why it is the preferred choice when both ends support it.
The catch is that T.38 needs support at the ATA, the provider and the far-end carrier. Many ATAs implement it badly, and some carriers refuse it or drop it between networks. A failed switch shows up as a SIP 488 or 415 response to the T.38 re-INVITE, or silence after the fax tones start.
| Method | Strength | Weakness |
|---|---|---|
| G.711 passthrough | Works anywhere with clean transport | Any loss or jitter hits the signal directly |
| T.38 relay | Resists loss and jitter | Needs support at every hop; poor interop is common |
How do I find out if the network or provider is the problem?
A fax that is fine at 2 a.m. and slow at 7 p.m. points to your uplink. Packet loss above about 1%, or jitter spikes, is enough to force fallback. Wi-Fi between the ATA and the router is a common culprit, so use Ethernet. Test while other devices are idle, and if you can, set QoS on the router to prioritize SIP (UDP 5060 or 5061) and RTP (the provider's range, often UDP 10000 to 20000).
- Send a plain text page to a different fax number, ideally with a different carrier. If it is fast to one number and slow to another, the far end or the carrier path is the cause.
- Check the fax machine's activity report or error code. Codes about line noise, no response, or a page that failed after retries confirm a signal problem rather than a paper or toner problem.
- Pause large uploads and streaming, wire the ATA, and retest. Improvement means congestion.
- Ask the provider for T.38 or G.711 passthrough on your line, or for a fax-optimized profile. Also ask whether their network transcodes calls between carriers, since each transcoding hop adds distortion.
- Be aware that some providers state fax is unsupported. In that case no setting is guaranteed to work.
; Asterisk/FreePBX operators can confirm what is happening on the SIP side.
; Run these from the Asterisk CLI or shell:
pjsip set logger on # look for T.38 re-INVITE and 488/415 replies
rtp set debug on # check for gaps or out-of-order packets
sngrep # (shell tool) watch the call flow live
; Sample pjsip.conf endpoint options for fax:
disallow=all
allow=ulaw
t38_udptl=yes
t38_udptl_ec=redundancy
t38_udptl_nat=yes
; Note: fax tone detection is not set on the endpoint.
; Handle it in the dialplan, for example by routing to a fax
; extension that uses ReceiveFAX() or by setting FAXOPT(ecm)=yes
; before SendFAX()/ReceiveFAX().When should I use an online fax service or an analog line instead?
If a plain text page still takes minutes after the Brother Basic (for VoIP) setting, a 9,600 bps cap, G.711 only, and a wired connection, stop tuning. The remaining cause is usually carrier transcoding or poor T.38 support, which you cannot control.
An online fax service such as eFax or Fax.Plus takes the fax over IP on a platform built for it, so your machine is no longer involved. This suits low volume and sending from email. A POTS line, or an analog line from the phone company, is the better fit when you need a physical machine, legal or medical documents with confirmed delivery, or a fax that must work during an internet outage.
Common questions
Is 14,400 bps or 9,600 bps better for fax over VoIP?
14,400 bps is faster when the line is clean, but 9,600 bps is more robust and often finishes sooner on a marginal line because it avoids repeated retraining. Try 14,400 first, and drop to 9,600 if the page fails or the speed keeps falling.
Why is my Brother fax not working on VoIP?
Brother machines default to a mode that tries V.34 Super G3 speeds, which VoIP often cannot carry. Set Compatibility to Basic (for VoIP), keep ECM on, and make sure the ATA fax port uses G.711 with echo cancellation and VAD off.
Can I use a fax machine with any VoIP provider?
Not reliably. Some providers explicitly exclude fax, and others support it only with T.38 or a specific ATA profile. Ask before relying on it, and test with several pages and different destination numbers.
Does a faster internet connection fix slow faxing?
Usually not. A G.711 fax call needs about 80 to 90 kbps including IP overhead, and T.38 needs much less, so the problem is packet loss, jitter and signal processing, not bandwidth. A faster plan helps only if congestion on your uplink was causing the loss.
Should I turn ECM off to make fax faster?
Generally no. ECM retransmits corrupted blocks, so pages arrive clean. Turning it off can shorten a transmission, but errors then print as lines or blocks and the fax may still be unreadable.