Fax over IP: getting a fax machine, router and VoIP line to work together
A configuration guide: keeping a fax machine in service when the line comes in over fibre, cable or an IP PBX.
Whether connected through a router or any VoIP line, a fax device depends on one essential requirement: its call must reach the other side without compression or added delay. Two options exist — G.711 passthrough, which treats the signal as an ordinary voice call, and T.38, which converts it into data packets. Restrict the machine to 9,600 bps, turn off ECM when using G.711, and validate the setup with test faxes covering multiple pages.
The key points
Throughout the entire session, the two modems interact: compression, packet loss, or jitter — any distortion of the audio disrupts the ongoing exchange, whether it concerns the live conversation or the document being sent.
T.38 is designed for IP networks, whereas G.711 passthrough can also function, provided the connection is clean and free of transcoding.
Starting point for the machine: top speed V.29 (9,600 bps), Super G3 mode (V.34) off, and no ECM unless the line runs T.38.
One page getting through tells you little. Before relying on the line, send something longer, several pages, and do it at a few different hours.
Fax over IP failure: the causes
In the copper era, fax machines ran on a dedicated line with stable latency. Once the signal passes through a router, it is digitized, broken into packets of 10–30 ms each, and carried over a network shared with other traffic. Yet T.30 expects particular tones at precise intervals: this architecture was never built to handle that.
Three factors explain the majority of outages. Codecs optimized for voice (G.729, Opus, AMR) deliberately discard the very details the modem depends on to transmit its payload. Jitter — packets that arrive unevenly — compels the router to stretch or cut the stream. And losing 20 ms at a critical instant can erase an entire scan line.
Error Correction Mode (ECM) is meant to help, yet over G.711 it is often the culprit. Every corrupted block is sent again; those repeats accumulate until the session misses its timing limits and the machine drops the call. With ECM disabled, a lost packet costs you, at most, a fine white line across an otherwise legible page.
G.711 passthrough or T.38: the showdown
The internet box or gateway sets the approach as soon as the call is initiated.
| Criterion | T.38 (fax relay) | G.711 in passthrough |
|---|---|---|
| Operation | As soon as the signal enters the IP network, it is demodulated, transmitted as data (UDPTL), and then remodulated when it leaves. | The modem noise travels raw, like an uncompressed conversation. |
| Lost packets | Reliable: every packet carries copies of earlier data (redundancy). | Poorly tolerated: a loss erases a line or cuts off the page. |
| Jitter (delay variation) | Limited impact: the modem timing is reconstructed locally. | Significant impact; a fixed-size jitter buffer helps a great deal. |
| Bandwidth | Low; it rises with the redundancy used. | Around 64 kbit/s in each direction for the audio; with IP headers, allow for 80 to 90 kbit/s. |
| Requirements | On the hardware side: the box or ATA, carrier, and remote gateway must all support the switchover to T.38 via a SIP re-INVITE. | G.711 throughout the chain (A-law in Europe), no transcoding. |
| Speed used | 14,400 bit/s (V.17) works most of the time; V.34 should still be avoided. | ECM disabled, speed capped at V.29 (9,600 bit/s). |
Settings: on the machine and in the box
Each brand names its menus its own way; use these values as a starting point.
- Modem speed
- Over IP, opening at 14,400 bit/s is a frequent cause of failed calls, so stick to V.29 and its 9,600 bit/s.
- Super G3 mode (V.34)
- Turn it off: 33,600 bit/s assumes a near-flawless line.
- ECM
- Turned off for G.711. When T.38 behaves reliably, re-enable it and run both modes side by side.
- The router's phone port
- Use the analog port (Tel, Phone, or Fon) connected directly — no splitter, no old ADSL filter. If the interface offers a fax mode, enable it.
- Codec and voice options
- If your router or ATA exposes these settings, give G.711 A-law top priority among codecs and disable echo cancellation, comfort noise and silence detection.
- Jitter buffer
- Prefer a fixed 60–100 ms buffer over an adaptive one, which recalibrates mid-call.
Which hardware: ATA, gateway, or fax server?
In Belgium, landlines almost always go through the operator's router — Proximus, on copper as on fibre; VOO and Telenet, on cable. Each one exposes an analog phone port, but each handles the fax call in its own way: G.711 passthrough, T.38 relay, or locked settings — it all depends on the box and its firmware. Look in your router's interface, ask the operator, and only then buy hardware.
An ATA turns an analog line into a SIP account — handy when your business takes its phone service from a VoIP provider or operates an IP PBX. Choose a unit that includes T.38 capability by default, then verify that your SIP trunk allows its use; otherwise, arrange a backup using G.711.
Several fax machines in the same building can share one gateway with multiple FXS ports. Going further, a fax server does away with the machines altogether: received faxes land as PDFs in users' mailboxes and staff send from their own computers. It only makes sense if you have a T.38 trunk plus someone whose job includes looking after it.
Test before concluding it works
Start by plugging a phone into the same port: is there a dial tone? Can you call a landline? If not, it's the line at fault, not the machine.
Next, transmit five to ten pages that combine fine text with dark solid blocks; ask the recipient to report exactly what arrived. Do this at three separate times, including an evening run when the network is busiest, and send one batch the other way to check incoming reception.
Keep the transmission reports: a device that consistently drops to 4,800 bit/s indicates a faulty line, even when pages still get through. Recurring issues? Contact your operator with the ticket, and confirm whether your line supports T.38.
Time to switch to online fax
Placing a fax machine behind the router makes sense when incoming faxes arrive on a fixed number. If you only send and do so only a handful of times a month, installation and upkeep end up costing more than the unit itself.
With an online service, you bypass the entire IP chain on your end: the PDF opens in the browser while the service dials the other party's fax number. That is exactly how Faxeroo operates, without creating an account, to reach landlines across seven countries, including Belgium. No number is published, no fax is received: it serves as an addition to your receiving line, nothing more.
Frequently Asked Questions: Fax over IP
- Will a fibre connection accept my fax machine?
- Yes: the fibre isn't at fault; the router or ATA determines the outcome. If G.711 passthrough is clean, or T.38 is in use, and the machine is limited to 9,600 bit/s, most sends succeed.
- Should ECM be turned off on a T.38 line?
- It isn't always the case. ECM typically only creates problems when the transmission is raw G.711. On a connection that actually negotiates T.38, leave ECM turned on, send a batch of test pages, and compare the outcome with another batch transmitted with ECM disabled.
- Does my router support T.38?
- Open the voice or telephony section of the router's settings: a setting labelled fax, or T.38, is the sign. If nothing like it appears, ask your operator's support, since it depends on the exact model and firmware version.
Telephony and networks
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- The equipment you need, the real cost, and why Faxeroo limits itself to sending.
- Not a relic, not a snobbery: a tool that solves one precise problem — establishing that a document arrived.
- From Bain's electric clock to the fax sent from a browser: a hundred and eighty years of image transmission.
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