modem performs a piece of telecoms history, then runs a real connection underneath it. Below: what you are hearing, phase by phase, and the standards actually implemented once CONNECT appears.
What you're hearing
Play the whole overture at once, rendered live by the same code that runs the connection - not a recording - or preview any stage on its own in the phase list below.
The nine phases
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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 most film and television dial tone actually 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 actually 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.