Museum of Portable Sound - The sounds of a dialup modem connecting to the internet are actually a language, and the sounds are a conversation between two 20th century-era machines. Listen along
Dial Up Modem Handshake Sound - Spectrogram - YouTube
Spectrogram Visualization of a Dial-Up Modem Handshake Sound
UZ7HO Sound Modem: Giving New Life To A "Dinosaur" - West Mountain Radio
Dial-up modem sounds analyzed and explained - The Verge
Jaton Modem-Sound Card New COMCL-SND 65C37CR2. COMCL+SOUND