I've just purchased a 33.6Kbps modem. I've just purchased a 56Kbps modem.
Modems negotiate a error free baud rate at connection time.
Because the mathematics of compressing 33.6kbps pushes the phone line to near its theoretical limits, V.34 was designed to accommodate a variety of phone line conditions. During the modem handshake, the modems send a series of tones to each other, at known signal levels and specific frequencies. The modem calculates the level of the received signal at each frequency, and therefore can determine the maximum bandwidth available for use. Three factors determine connecting at 56kbps. Your phone line must be capable of supporting K56 speeds.