The dial-up sound was never one noise. It was a sequence, and every part of it was doing a job. Below: the whole dial-up modem handshake, phase by phase, what each sound was for, and the standards this project implements once CONNECT appears.
Why did dial-up make that noise at all? Because two modems meeting over a voice line had to find each other, agree how to speak, and measure what the line could carry - out loud, because sound was the only channel they had.
Common questions about the dial-up sound
What is the dial-up sound?
It's the performed overture a modem plays before a call connects: the click of the line being seized, a dial tone, DTMF digits, a ringback tone, then the answer tone and its phase reversals. modem renders this live in the browser with the same code that runs its real connection, then completes a genuine 300 baud Bell 103 connection underneath it.
Why did dial-up make that noise?
Each sound had a job. The dial tone said the line was ready; DTMF sent the digits; ringback meant the far end's phone was ringing; the 2100 Hz answer tone, with its phase reversed every 450 milliseconds, told the phone network's echo suppressors to switch off so two modems could talk over each other without being muted.
What is that screeching noise at the start of a dial-up call?
On a real 33.6k or 56k modem it was V.34-style probing: two modems measuring the line's own noise and frequency response before agreeing a speed. modem performs an impression of that stage for texture - there is no physical phone line here to measure, so it is the one stage on this page that is not a faithful reconstruction of a real signal.
Is the dial-up sound on this page a recording?
No. The overture is a reconstruction rendered live by the same code that drives the real connection, not a recording - and the connection itself is a genuine 300 baud Bell 103 link, not simulated.
What is a Bell 103 modem?
AT&T's 1963 300 baud modem standard - the connection this project actually implements once CONNECT appears, with the same originate and answer tone pairs, 1270/1070 Hz and 2225/2025 Hz.
What is FSK modulation?
Frequency-shift keying: a bit is a shift between two frequencies. It's the modulation underneath the connection itself, once the overture ends and CONNECT appears.
What is a modem handshake?
The sequence two modems perform before data flows: seizing the line, exchanging tones to prove they are both modems, agreeing capabilities, then settling into a stable connection. modem's own explainer plays each phase of that handshake individually.
What the dial-up sound actually is
Play the whole dial-up sound at once, generated live by the same code that runs the connection - these are not recordings or sound effects - or hear any single stage on its own in the phase list below.
The nine phases of the handshake
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Off hook
The click of the line being seized - a relay closing, not a tone. It is the sound of the call existing at all, a beat before anything is dialled.
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Dial tone
A continuous two-tone signal telling the exchange the line is ready to take a number. The UK pair is 350 and 450 Hz, different from the US pair that most film and television dial tone is.
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Dialling
DTMF, the touch-tone digits. Each key sends two frequencies at once, one from a row and one from a column, so the exchange can tell exactly which one was pressed.
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Ringback
The ringing tone you hear while the far end's phone rings, generated by your own exchange rather than the far end's phone. The UK cadence is a double ring: two short bursts of 400 and 450 Hz, then a pause.
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CI
The calling indicator. Before anything else happens, the originating modem announces itself on a channel a voice call never touches, so the far end knows straight away that this is a data call.
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ANSam
The answer tone: 2100 Hz, its phase flipped 180 degrees every 450 milliseconds. Analogue phone networks ran echo suppressors that muted a call whenever it looked like only one side was talking, fine for speech and fatal for two modems talking at once. The phase reversals are a signal those suppressors were built to recognise, and switch themselves off for.
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CM/JM
Call menu and joint menu. The two modems compare what modulations they each support and settle on the one they will use.
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CJ
The acknowledgement that both ends agreed, followed by a short, deliberate silence: the last quiet moment before the two modems start probing the line itself.
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Training
An impression of the real V.34-style probing that follows in an actual call - the fast, noisy screech most people actually remember from a 33.6k or 56k dial-up modem - where two modems measure the line's own noise and frequency response before committing to a speed. There is no physical phone line here to measure, so this stage is texture, not a conformant negotiation - the one stage on this page that is not a faithful reconstruction.
Bell 103 and FSK after CONNECT
Once CONNECT appears, the overture is over and a genuine 300 baud Bell 103 connection takes over - AT&T's 1963 300 baud modem standard, with the same originate and answer tone pairs, 1270/1070 Hz and 2225/2025 Hz. The modulation underneath it is FSK: frequency-shift keying, a bit is a shift between two frequencies, nothing more exotic than that.