Once upon a time I worked for what was (once) the world's largest BBS. Remember the US Robotics Courier HST? 16.8kbps! Now imagine a room filled with shelf after shelf of Couriers, several hundred individual lines, at least a few hundred amps worth - all negotiating vigorously, clicking on-hook and off, all the time. Fun times.
Pretty sure I could still tell the difference between a 14.4k and 16.8k negotiation by ear.