What this is
modem runs a real Bell 103 connection between two machines over their sound cards. Getting there takes a moment: a faithfully performed dial-up overture goes first - off-hook, dial tone, DTMF, ringback, the ANSam answer tone and its phase reversals - then a real 300 baud connection underneath.
What's performed, and what's real
The overture is a reconstruction. The FSK connection underneath it is real.
Every phase of that overture, including the one stage that only performs an impression of a real signal rather than reconstructing one, is explained phase by phase on its own page.
Who made it, and why
Daniel Grimes built it, at DBHQ. Every crate in it - modem-core, modem-audio, modem-tui, modem-wasm - and this page are open source under the MIT licence, linked at the foot of every page.
The actual reason it exists is Oona Raisanen's annotated dial-up handshake spectrogram, published on Hackaday - every stage of the sound named against what was actually making it. The full citation, alongside the rest of the research and the wider history, is on its own page.