What the fax machine sound means
The fax machine sound is two fax machines talking over a phone line. The calling machine beeps to say it's a fax, and the fax machine that answers whistles back. Then they trade bursts of warbling to agree on speed and settings, and the page itself goes as a steady hiss.
Each part of that noise is a signal from ITU-T Recommendation T.30, the international rulebook for fax calls, or from the modem standards it uses. The table below lists them in the order you hear them.
Each part of the sound, in the order you hear it
The table follows a normal send, in which the machine that dials sends the page. The standards give the signals, not how they sound, so the "What you hear" column is our description.
| Order | What you hear | Signal and standard | What it means |
|---|---|---|---|
| 1 | A soft beep, half a second long, then 3 seconds of quiet, over and over | Calling tone (CNG), 1100 Hz, T.30 | The caller is a fax machine, not a person. |
| 2 | A steady high whistle for about 3 seconds | Answer tone (CED, for called terminal identification), 2100 Hz, T.30 | A fax machine has answered the line. |
| 3 | A rough, buzzy warble | Called subscriber identification (CSI) if sent, then the digital identification signal (DIS), at 300 bits per second with the V.21 modem | The answering fax machine may give its own number, then lists what it can do. |
| 4 | The same warble, from the other end | Transmitting subscriber identification (TSI) if sent, then the digital command signal (DCS), V.21 | The caller may identify itself, then picks the speed, resolution and paper size for this fax. |
| 5 | A short hiss | Modem training, then the training check (TCF), at the chosen speed | The caller tests the line at the high speed for 1.5 seconds. |
| 6 | A quick warble | Confirmation to receive (CFR) or failure to train (FTT), V.21 | The receiving machine says "ready" (CFR), or says the line test failed and asks for a retrain (FTT). |
| 7 | A long, steady hiss | The page, sent with V.17, V.29 or V.27 ter | The coded page goes across. |
| After each page | Short warbles back and forth | Multipage signal (MPS) or end of procedure (EOP), then message confirmation (MCF) or retrain negative (RTN), V.21 | "More pages follow" or "that was the last," then "got it" or "not received well." |
| The end | One last warble, then the line goes quiet | Disconnect (DCN), V.21 | One machine tells the other the call is over. |
Brother's owner's manual for the FAX-4100e, 4750e and 5750e describes the same thing in plain words. It calls the calling tones "soft, intermittent beeps" at 4-second intervals that go on for about 60 seconds after dialing. By our arithmetic, that spacing fits within the 15% timing tolerance T.30 allows. The receiving machine's tones are "loud, chirping sounds," and Brother calls the stretch where the two overlap the handshake.
Why the warble and the hiss sound different
All the short messages, from the list of features to the final goodbye, normally go at only 300 bits per second with the V.21 modem. It sends each bit as one of two tones, 1650 Hz for a one and 1850 Hz for a zero. As we understand it, that flicking between two pitches is the warble you hear. Each burst opens with about a second of the same repeating flag pattern, so the warbles tend to start alike.
By our arithmetic, the page goes up to 48 times faster, at up to 14,400 bits per second with V.17. That modem uses quadrature amplitude modulation on an 1800 Hz carrier tone at 2,400 symbols a second. In plain words, it changes the strength and phase of that tone 2,400 times a second. It also runs the data through a scrambler first.
In our reading, scrambled data has no steady pattern to hear as a pitch, so the page comes across as hiss. The receiving machine's modem undoes the scrambling and reads the bits back out.
The hiss carries the page as code, not as a picture. Our guide to how a fax machine works explains how the scanner turns a page into that code and how the other machine prints it.
V.34 fax machines can answer with a different tone
Fax machines that support the faster V.34 modem, up to 33,600 bits per second, can use a different answer tone and setup. The answering fax machine sends a modified answer tone called ANSam, the same 2100 Hz pitch with its loudness pulsing 15 times a second. When echo cancellers on the line need turning off, the wave also flips every 450 milliseconds. The two machines then swap short menus of what they support, called CM and JM in Recommendation V.8, before the V.34 start-up and the page.
All of these signals belong to Group 3, the fax standard first recommended in 1980 and since extended. The ITU still lists T.30 as in force. Our history guide explains how Group 3 came about.
Need to send a fax without hearing any of this? This website faxes your PDFs or photos to a fax number in the US or Canada, and you don't need an account. For paper pages, the FaxFair iPhone app scans them with the camera.
Send a fax onlineQuestions
What does it mean when a fax machine calls my phone?
It means a fax machine or fax service is dialing your number to send a fax. A beep every few seconds is the calling tone, which T.30 says marks a call from a machine, not a person. A fax only goes through when a fax machine or fax service answers, as our guide to what a fax number is explains. If the calls keep coming, you can block the number.
- On an iPhone, open the Phone app and tap Recents. Tap the number (in Classic view, tap the Info button instead), then scroll down and tap Block Caller.
- In the Phone by Google app, tap More, then Call history. Tap a call from the number, then tap Block / report spam. Google says some of these steps only work on Android 6.0 and up.
Why do fax machines make that noise?
Because a fax travels over a phone line as sound. The beeps and whistles tell each side that a fax machine is on the line. The warbles carry the settings, and the hiss is the page itself, sent as code by a modem.
What does it sound like when you call a fax machine?
You hear a steady high whistle, the 2100 Hz answer tone, and then bursts of warbling. As we understand it, the warbles are the machine listing its settings for a fax machine that never replies. One Brother owner's manual says its machine chirps for about 60 seconds over the phone line.
Why is there a fax sound on my voicemail?
Most likely a fax machine or fax service called you and your voicemail recorded its calling tone, a half-second beep every 3.5 seconds. Brother's manual says those beeps go on for about 60 seconds after dialing. You can block the number, as in the first answer above. On an iPhone a blocked caller can still leave a voicemail, but Apple says you won't get a notification for it. Google says that in its Phone app, if visual voicemail is on, blocked callers cannot leave voicemails.
Sources
The pages we read for this guide.
- ITU-T, Recommendation T.30 (09/2005), "Procedures for document facsimile transmission in the general switched telephone network"
- ITU, "T.30: Procedures for document facsimile transmission in the general switched telephone network"
- ITU-T, Recommendation V.21 (11/88), "300 bits per second duplex modem standardized for use in the general switched telephone network"
- ITU-T, Recommendation V.17 (02/91), "A 2-wire modem for facsimile applications with rates up to 14 400 bit/s"
- ITU-T, Recommendation V.8 (11/2000), "Procedures for starting sessions of data transmission over the public switched telephone network"
- IEEE Milestone, "International Standardization of G3 Facsimile, 1980"
- Brother, FAX-4100e, FAX-4750e and FAX-5750e Owner's Manual (USA)
- Apple Support, "Block phone numbers, contacts, and emails on your iPhone or iPad"
- Google, Phone app Help, "Block or unblock a phone number"