Fax Number and Phone System: Routing Calls the Right Way

Short answer: in 2026, making voice and fax share a single number is the number-one cause of lost faxes in small and mid-sized businesses. Only three configurations work reliably: a dedicated fax number (ideally an online fax number, with no hardware), a separate DID on the company's SIP trunk, or — as a last resort — CNG tone detection by the phone system, which automatically switches the call over to the fax service. Everything else (the classic "press 2 to send a fax", manual forwarding, sharing a line between the reception phone and the fax machine) produces the same symptoms: calls answered by a human greeted with a modem tone, documents that never arrive, senders who give up after two attempts.

The topic seems trivial. It isn't: since the French network migrated to IP telephony, the phone system is no longer a box sitting in a cupboard but a service configured through a web interface, and the routing rules that worked on an analogue line don't carry over as-is. A "no answer" conditional forward that worked fine in 2015 can today cut off the T.30 negotiation before it even begins. The result: nobody reports a fault — since the phone rings normally — and faxes disappear silently.

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Why a shared voice/fax line fails over IP

On a traditional analogue line, voice/fax sharing relied on a simple principle: the caller dialled the number, the remote fax machine emitted a CNG tone (a burst of 1,100 Hz every three seconds), and a small "fax/phone switch" box detected that burst to route the call. It worked reasonably well because the line carried a continuous, uncompressed audio signal.

With VoIP, three things change:

• Compression. Many carriers default to G.729 or G.711 with silence suppression. G.729 is optimised for the human voice: it distorts pure tones. A CNG burst run through G.729 can become undetectable.
• Decision time. A CNG detector needs to hear at least one or two bursts before making a call, i.e. 3 to 6 seconds. Meanwhile, the remote fax machine is counting its attempts: most give up after 35 to 45 seconds of failed negotiation.
• The path. An IP call forward often re-establishes a new media session. If T.38 isn't renegotiated correctly on that second leg, the fax falls back to G.711 "passthrough", with all the jitter and packet-loss risks that entails. We covered this mechanism in detail in our guide to T.38 and fax failures over VoIP.

In other words: sharing voice and fax on a single number was already a workaround in the analogue era; over IP, it's a gamble.

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The four routing scenarios, from best to worst

1. A dedicated fax number, hosted online

This is by far the most robust setup. The fax number is no longer connected to a device but to a service: the inbound call is terminated on the carrier side, T.30/T.38 negotiation happens on its infrastructure, and the document arrives as a PDF in an email inbox or a shared space. No forwarding, no detection, no hardware to maintain.

Concrete advantages:

• the number rings 24/7, even when the office is closed;
• several recipients can receive the same fax (front desk + practitioner, accounting + management);
• the send and receive log is timestamped and exportable;
• no more paper piling up in an output tray.

This is the principle described in detail in our guide to receiving faxes by email. For most organisations with fewer than twenty people, it's the default answer in 2026.

2. A dedicated DID on the SIP trunk

If the company already has a SIP trunk with a block of numbers (DIDs), setting one aside for fax generally costs a few euros a month. The phone system routes that DID to an ATA gateway or a fax server, without ever presenting the call to a voice extension.

Points to watch:

• force G.711a codec (alaw in France) on that DID only, and disable silence suppression and echo cancellation;
• enable T.38 with G.711 fallback if your carrier supports it;
• don't place any announcement, hold music or legal message before the call is answered: every second consumed shrinks the negotiation window;
• set the ring count to one — or better still, immediate answer.

If you're connecting a physical fax machine to the network, the adapter matters as much as the settings: a T.38-compatible ATA adapter from a reputable brand saves you weeks of troubleshooting, whereas a budget box will settle for rough-and-ready passthrough.

3. Automatic tone detection (fax detect)

Most modern IP PBXs — 3CX, Asterisk/FreePBX, Wildix, and the cloud phone systems offered by French carriers — provide a "fax detection" feature: the system answers, listens for a few seconds, and switches the call to the fax queue if it hears a CNG tone.

It works, but under strict conditions:

And above all: detection only works if the remote fax machine actually emits a CNG tone, which isn't guaranteed with manual sending (an operator who dials and then presses "Start" often skips that phase).

4. The "press 2 for fax" IVR

This is the most widespread and the worst configuration. An interactive voice response system asks the caller to press a key; but here the caller is a machine, which will never listen to the announcement and will never press anything. An IVR only works when a human dials the number from a fax machine's handset before pressing "Start" — a practice that has become vanishingly rare.

If you inherit such a setup, migrating to a dedicated number solves the problem in a single day.

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The case of call forwarding

Forwarding is the most insidious trap. Three forms coexist, with very different effects on fax traffic:

Unconditional forwarding (CFU). All calls go elsewhere immediately. This is the least dangerous: the session is established once and for all to the final destination. If that destination is a fax service, it works.

Forward on no answer (CFNR). The phone rings five times, then the call switches over. For a fax, those five rings are five seconds lost, and the switchover triggers a media renegotiation. High failure rate.

Forward on busy (CFB). Behaviour varies unpredictably between carriers; some briefly interrupt the RTP stream, which is enough to fail a page in progress.

The practical rule: a fax number must never depend on conditional forwarding. If forwarding is absolutely necessary, choose the unconditional kind.

Also worth noting: forwarding to a mobile. A fax forwarded to a mobile phone will never arrive: the French mobile network hasn't carried fax over the voice circuit for a long time, and no smartphone demodulates a T.30 signal. It's a classic mistake when practices or firms close over the summer.

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Organising the human side: who receives what

Technical routing is pointless if nobody knows what to do with the document. In an organisation with a front desk, accounting and management, the question "which address do faxes land in?" should be handled as a business rule, not a checkbox.

Three arrangements work well:

• A single shared inbox (fax@company.com), monitored by two people, with automatic filing rules by sender. Simple, but it assumes disciplined processing.
• One address per department, with a separate number per department. The clearest option for external correspondents, especially if the company covers several lines of business (an accounting firm receiving both client documents and bailiff letters has no interest in mixing everything together).
• Automatic filing into the document management system, with email notification. This is the target setup for organisations that already have a document platform.

In every case, plan for comfortable on-screen reading: sorting twenty faxes a day on a 13-inch laptop is nerve-racking, whereas a 24-inch secondary monitor in portrait orientation displays a full A4 page without zooming. And if you still need to feed signed paper documents back into the workflow, a duplex desktop scanner with an automatic feeder saves more time than any IP PBX setting ever will.

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The ten-minute verification checklist

Before concluding that "there's no problem", test:

1. A test send from an external number (not from an internal extension: the path isn't the same).
2. A test outside business hours, in the evening or at the weekend, to check that night-time forwarding doesn't break anything.
3. A document of at least three pages: many setups get page 1 through and then fail.
4. An international send, if you receive documents from abroad — timeouts vary from country to country, as detailed on our supported countries page.
5. A review of the log: a fax "received" with 0 pages or an error code 232 signals an interrupted negotiation, not a sender error.

If the test fails, work back up the chain in this order: codec → announcement/timing → forwarding → terminal equipment. Nine times out of ten, the culprit is in the first two links.

For sites that keep a physical fax machine as a backup, don't forget the UPS: an IP phone system and a router down for thirty seconds is enough to fail an incoming fax, and a small backup power supply — a UPS for your router — protects the whole setup. Also keep a supply of thermal paper rolls or compatible toner on hand: nothing is more irritating than a document that arrived but can't be printed on a Friday evening.

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Frequently asked questions

Can you keep a single number for voice and fax in 2026?

Technically yes, with well-configured CNG detection and a forced G.711 codec. In practice, the failure rate remains around 10 to 30% depending on the senders' carriers, and you'll never know which documents failed to arrive. For any serious professional use, a dedicated number is the only defensible choice.

My cloud phone system offers "fax to email": is that enough?

Often yes, provided you check three things: is the DID exclusively dedicated to fax, is the output format a usable PDF (and not a low-resolution image), and is the log exportable? Some bundled offers cap the resolution or only archive for 30 days.

What if my correspondent only has my old number?

Keep the old number on unconditional forwarding to the new service while you spread the word. Allow at least six months: fax numbers circulate in internal directories, letterheads and rubber stamps that get updated very slowly. Number portability is covered in our guide to keeping your number after the PSTN switch-off.

Do I need a second physical line?

No. In 2026, a second number no longer means a second copper line: on a SIP trunk it's an additional DID; with an online fax service it's simply a software assignment. The cost bears no comparison to an analogue subscription of the old days.

How do I know if a fax failed at the sender's end?

You won't — and that's precisely the problem. The sending fax machine prints a failure report on its side; you see nothing. Hence the importance of regular testing and checking the receive log, rather than blindly trusting the silence.

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Key takeaways

• Sharing voice and fax on a single number is the #1 cause of lost faxes in small businesses.
• Four configurations, in order of reliability: dedicated online fax number > dedicated DID on a SIP trunk > CNG detection > keypad IVR (to be avoided entirely).
• On the fax DID: force G.711a, enable T.38, zero announcements, immediate answer.
• No conditional forwarding on a fax number; never forward to a mobile.
• Define the human recipient (shared inbox, per-department address or document management system) before touching the routing.
• Test from outside, outside business hours, with three pages and internationally — then read the log.

You can test a send right now from the send form or check the frequently asked questions for special dialling cases.