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That noise is two machines talking. The answering machine plays a steady tone, the caller answers with its own beep, and then they trade short bursts of test signals to agree on a speed and protocol. The whole screech lasts only a few seconds, and it is why a fax number sounds nothing like a voicemail.
You only hear it because fax was designed for the voice network. The machines have to hold their conversation inside an ordinary phone call, using sounds, because the phone line cannot tell the difference between a human voice and a modem. So the machines literally speak to each other, and you get to listen in.
The call opens with the caller beeping: a short 1100 Hz tone repeated every few seconds that says "I am a fax machine calling." When the other end answers as a fax, it responds with a longer, steady 2100 Hz tone that says "I am a fax machine answering." Those two tones identify both parties.
Then comes the part that sounds like static from a broken radio. That is the training sequence: the machines exchange known test patterns so each can measure how badly the phone line distorts sound, then agree on the fastest speed the line can handle cleanly. A clean digital line settles quickly; a noisy long-distance line takes longer and settles slower. After that the line usually goes quiet, because most machines mute their speakers once the actual document starts moving.
| What you hear | What it means |
|---|---|
| Short beeps, evenly spaced | A fax machine is calling out and has not connected yet |
| One long steady tone | A fax machine answered and is identifying itself |
| Harsh warbling and static | The two machines are testing the line and agreeing on a speed |
| Sudden silence | The handshake finished; the document is transmitting (or the speaker muted) |
| The warbling restarts mid-call | The machines renegotiated, usually because line quality dropped |
Because audibility was the debugging tool. In the 1980s, when you heard that sound, you knew the call had reached a fax machine and not a person, and the pattern of the noise told a technician roughly where a failed call had broken down. The speaker on the machine was left on during dialing and connecting on purpose, the same way old modems screeched through their speakers. Once the connection was up, the speaker's job was done.
There is a second reason it sounds harsh: the test signals are designed to stress the line, not to please the ear. They pack as much information as possible into the narrow slice of frequencies a phone call can carry, and the result is about as musical as a fire alarm.
The machines try again, a few times, and then give up. You get the classic symptoms: the call connects, screeches for a while, and dies, or the sender's machine reports a communication error. Common causes are a line with too much noise (long-distance calls over cheap VoIP routes are notorious), one side using a very old machine that only speaks an ancient protocol, or an answering machine picking up first and confusing the caller.
Here is the non-obvious one: some VoIP adapters compress voice aggressively to save bandwidth, and that compression mangles fax tones beyond recognition. If faxes fail consistently over a particular internet phone line but work over a mobile call to the same number, the VoIP compression is the likely culprit, and switching the adapter to a fax-friendly codec (or using an online fax service that never touches the voice network) fixes it.
Not usefully. People have tried whistling the answer tone to trick a calling fax machine, and in theory the machine would start its handshake with you standing there, but there is nothing you could do with the conversation after that: the protocol that follows is digital data at speeds no human can imitate. The practical version of this question comes up with robocalls: if your phone rings and you hear the fax screech, someone (or something) dialed your number expecting a fax machine. Hang up. It is either a misdial or a machine dialing a stale list.
No. The screech is a property of fax machines with speakers, not of faxing itself. Online fax services, including LetterAgent's fax flow (currently in testing), send your document as data over their own connections: you describe the fax to your AI assistant, approve the quoted total, and it is sent. You will never hear a tone. The handshake still happens, deep inside the provider's equipment, but it happens silently, the way it should have all along.
Ready to fax it? Fax with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is sent.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
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