To add further confusion, 57600 was actually a serial port speed, from the computer to the modem, which was higher than the maximum physical line (modem) speed. Many people ran higher serial port speeds to take advantage of compression (115200 was common.)

56000 BPS was the bitrate you could get out of a DS0 channel, which is the digital version of a normal phone line. A DS0 is actually 64000 BPS, but 1 bit out of 8 is "robbed" for overhead/signalling. An analog phone lined got sampled to 56000 BPS, but lines were very noisy, which was fine for voice, but not data.

7 bits per sample * 8000 samples per second = 56000, not 57600. That was theoretical maximum bandwidth! The FCC also capped modems at 53K or something, so you couldn't even get 56000, not even on a good day.