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
modem is a DBHQ experiment by Daniel Grimes. The full workspace - modem-core, modem-audio, modem-tui, modem-wasm and this page - lives on GitHub, MIT licensed.
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 prior art and the wider history, is on its own page.