For what it's worth, when I was on dial up, I was downloading a file from some univeristy's server and it was about 3am in the morning. The transfer speed was a steady 12.5 kilobyte all the way through the 20 some-odd MB file. 56K modems do have the capability of having high transfer speeds, it all depends on the physical location and quality of the connection from client/server.
Also, in reality normal phone lines can only transfer at 28.8 kbps (kilobits). To acheive the higher transfer speeds, TCP/IP compression is used. Packets are compressed when sending, decompressed when receiving. Hardware modems are very good at this which is why you'll get better performance out of them as opposed to crappy "winmodems" that rely on the main CPU for (de)compression.